build: fix regression in requiring yajl for new enough qemu

Jonathan Lebon reported an issue to me off-list about his build
failing to use qemu because he failed to install yajl-devel.  But
I recalled specifically tweaking configure.ac to die in that
situation (commits 350583c8, ba9c38b4).  After a bit more
head-scratching, we found the cause of the regression: commit
654c709b rearranged things so that the qemu version check now
occurs before AC_ARG_WITH has had a chance to set either
$with_qemu or $with_yajl.

Coincidentally, this fix aligns with a documentation patch that
was just posted to the autoconf mailing list :)
http://thread.gmane.org/gmane.comp.sysutils.autoconf.patches/8324

* m4/virt-lib.m4 (LIBVIRT_CHECK_LIB, LIBVIRT_CHECK_LIB_ALT)
(LIBVIRT_CHECK_PKG): Populate defaults earlier.
* configure.ac (AC_ARG_WITH): Likewise for drivers.
Signed-off-by: NEric Blake <eblake@redhat.com>

configure.ac

@@ -432,77 +432,77 @@ AM_CONDITIONAL([WITH_FREEBSD], [test "$with_freebsd" = "yes"])
 dnl Allow to build without Xen, QEMU/KVM, test or remote driver
 AC_ARG_WITH([xen],
   [AS_HELP_STRING([--with-xen],
-    [add XEN support @<:@default=check@:>@])],
-  [],[with_xen=check])
+    [add XEN support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_xen=check])
 AC_ARG_WITH([xen-inotify],
   [AS_HELP_STRING([--with-xen-inotify],
-    [add XEN inotify support @<:@default=check@:>@])],
-  [],[with_xen_inotify=check])
+    [add XEN inotify support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_xen_inotify=check])
 AC_ARG_WITH([qemu],
   [AS_HELP_STRING([--with-qemu],
-    [add QEMU/KVM support @<:@default=yes@:>@])],
-  [],[with_qemu=yes])
+    [add QEMU/KVM support @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_qemu=yes])
 AC_ARG_WITH([uml],
   [AS_HELP_STRING([--with-uml],
-    [add UML support @<:@default=check@:>@])],
-  [],[with_uml=check])
+    [add UML support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_uml=check])
 AC_ARG_WITH([openvz],
   [AS_HELP_STRING([--with-openvz],
-    [add OpenVZ support @<:@default=check@:>@])],
-  [],[with_openvz=check])
+    [add OpenVZ support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_openvz=check])
 AC_ARG_WITH([vmware],
   [AS_HELP_STRING([--with-vmware],
-    [add VMware support @<:@default=yes@:>@])],
-  [],[with_vmware=yes])
+    [add VMware support @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_vmware=yes])
 AC_ARG_WITH([phyp],
   [AS_HELP_STRING([--with-phyp],
-    [add PHYP support @<:@default=check@:>@])],
-  [],[with_phyp=check])
+    [add PHYP support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_phyp=check])
 AC_ARG_WITH([xenapi],
   [AS_HELP_STRING([--with-xenapi],
-    [add XenAPI support @<:@default=check@:>@])],
-  [],[with_xenapi=check])
+    [add XenAPI support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_xenapi=check])
 AC_ARG_WITH([libxl],
   [AS_HELP_STRING([--with-libxl],
-    [add libxenlight support @<:@default=check@:>@])],
-  [],[with_libxl=check])
+    [add libxenlight support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_libxl=check])
 AC_ARG_WITH([vbox],
   [AS_HELP_STRING([--with-vbox=@<:@PFX@:>@],
-    [VirtualBox XPCOMC location @<:@default=yes@:>@])],
-  [],[with_vbox=yes])
+    [VirtualBox XPCOMC location @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_vbox=yes])
 AC_ARG_WITH([lxc],
   [AS_HELP_STRING([--with-lxc],
-    [add Linux Container support @<:@default=check@:>@])],
-  [],[with_lxc=check])
+    [add Linux Container support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_lxc=check])
 AC_ARG_WITH([esx],
   [AS_HELP_STRING([--with-esx],
-    [add ESX support @<:@default=check@:>@])],
-  [],[with_esx=check])
+    [add ESX support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_esx=check])
 AC_ARG_WITH([hyperv],
   [AS_HELP_STRING([--with-hyperv],
-    [add Hyper-V support @<:@default=check@:>@])],
-  [],[with_hyperv=check])
+    [add Hyper-V support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_hyperv=check])
 AC_ARG_WITH([parallels],
   [AS_HELP_STRING([--with-parallels],
-    [add Parallels Cloud Server support @<:@default=check@:>@])],
-  [],[with_parallels=check])
+    [add Parallels Cloud Server support @<:@default=check@:>@])])
+m4_divert_text([DEFAULTS], [with_parallels=check])
 AC_ARG_WITH([test],
   [AS_HELP_STRING([--with-test],
-    [add test driver support @<:@default=yes@:>@])],
-  [],[with_test=yes])
+    [add test driver support @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_test=yes])
 AC_ARG_WITH([remote],
   [AS_HELP_STRING([--with-remote],
-    [add remote driver support @<:@default=yes@:>@])],
-  [],[with_remote=yes])
+    [add remote driver support @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_remote=yes])
 AC_ARG_WITH([libvirtd],
   [AS_HELP_STRING([--with-libvirtd],
-    [add libvirtd support @<:@default=yes@:>@])],
-  [],[with_libvirtd=yes])
+    [add libvirtd support @<:@default=yes@:>@])])
+m4_divert_text([DEFAULTS], [with_libvirtd=yes])
 AC_ARG_WITH([chrdev-lock-files],
   [AS_HELP_STRING([--with-chrdev-lock-files],
     [location for UUCP style lock files for character devices
-     (use auto for default paths on some platforms) @<:@default=auto@:>@])],
-  [],[with_chrdev_lock_files=auto])
+     (use auto for default paths on some platforms) @<:@default=auto@:>@])])
+m4_divert_text([DEFAULTS], [with_chrdev_lock_files=auto])
 
 dnl
 dnl in case someone want to build static binaries

m4/virt-lib.m4

@@ -62,10 +62,10 @@ AC_DEFUN([LIBVIRT_CHECK_LIB],[
   m4_pushdef([arg_var], [with-]check_name_lc)
   m4_pushdef([with_var], [with_]check_name_lc)
 
+  m4_divert_text([DEFAULTS], [with_var][=check])
   AC_ARG_WITH(check_name_lc,
     [AS_HELP_STRING([--arg_var],
-                    [with lib]]m4_dquote(library_name)[[ support @<:@default=check@:>@])],
-    [],[with_var][=check])
+                    [with lib]]m4_dquote(library_name)[[ support @<:@default=check@:>@])])
 
   old_LIBS=$LIBS
   old_CFLAGS=$CFLAGS
@@ -192,10 +192,10 @@ AC_DEFUN([LIBVIRT_CHECK_LIB_ALT],[
   m4_pushdef([config_var_alt], [WITH_]check_name_alt)
   m4_pushdef([make_var_alt], [WITH_]check_name_alt)
 
+  m4_divert_text([DEFAULTS], [with_var][=check])
   AC_ARG_WITH(check_name_lc,
     [AS_HELP_STRING([--arg_var],
-                    [with lib]]m4_dquote(library_name)[[ support @<:@default=check@:>@])],
-    [],[with_var][=check])
+                    [with lib]]m4_dquote(library_name)[[ support @<:@default=check@:>@])])
 
   old_LIBS=$LIBS
   old_CFLAGS=$CFLAGS
@@ -318,10 +318,10 @@ AC_DEFUN([LIBVIRT_CHECK_PKG],[
   m4_pushdef([arg_var], [with-]check_name_lc)
   m4_pushdef([with_var], [with_]check_name_lc)
 
+  m4_divert_text([DEFAULTS], [with_var][=check])
   AC_ARG_WITH(check_name_lc,
     [AS_HELP_STRING([--arg_var],
-                   [with ]]m4_dquote(pc_name)[[ (>= ]]m4_dquote(pc_version)[[) support @<:@default=check@:>@])],
-    [],[with_var][=check])
+                   [with ]]m4_dquote(pc_name)[[ (>= ]]m4_dquote(pc_version)[[) support @<:@default=check@:>@])])
 
   fail=0
   if test "x$with_var" != "xno" ; then