configure: Check if struct fsxattr is available from linux header

Fixes build failure with --enable-xfsctl and
new linux headers (>=4.5) and older xfsprogs(<4.5):
In file included from /usr/include/xfs/xfs.h:38:0,
                 from /var/tmp/portage/app-emulation/qemu-2.5.0-r1/work/qemu-2.5.0/block/raw-posix.c:97:
/usr/include/xfs/xfs_fs.h:42:8: error: redefinition of ‘struct fsxattr’
 struct fsxattr {
        ^
In file included from /var/tmp/portage/app-emulation/qemu-2.5.0-r1/work/qemu-2.5.0/block/raw-posix.c:60:0:
/usr/include/linux/fs.h:155:8: note: originally defined here
 struct fsxattr {

This is really a bug in the system headers, but we can work around it
by defining HAVE_FSXATTR in the QEMU headers if linux/fs.h provides
the struct, so that xfs_fs.h doesn't try to define it as well.

CC: qemu-trivial@nongnu.org
CC: Markus Armbruster <armbru@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Stefan Weil <sw@weilnetz.de>
Tested-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NJan Vesely <jano.vesely@gmail.com>
[PMM: adjusted commit message, comments]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

configure

@@ -4493,6 +4493,21 @@ if test "$fortify_source" != "no"; then
   fi
 fi
 
+##########################################
+# check if struct fsxattr is available via linux/fs.h
+
+have_fsxattr=no
+cat > $TMPC << EOF
+#include <linux/fs.h>
+struct fsxattr foo;
+int main(void) {
+  return 0;
+}
+EOF
+if compile_prog "" "" ; then
+    have_fsxattr=yes
+fi
+
 ##########################################
 # End of CC checks
 # After here, no more $cc or $ld runs
@@ -5160,6 +5175,14 @@ fi
 if test "$have_ifaddrs_h" = "yes" ; then
     echo "HAVE_IFADDRS_H=y" >> $config_host_mak
 fi
+
+# Work around a system header bug with some kernel/XFS header
+# versions where they both try to define 'struct fsxattr':
+# xfs headers will not try to redefine structs from linux headers
+# if this macro is set.
+if test "$have_fsxattr" = "yes" ; then
+    echo "HAVE_FSXATTR=y" >> $config_host_mak
+fi
 if test "$vte" = "yes" ; then
   echo "CONFIG_VTE=y" >> $config_host_mak
   echo "VTE_CFLAGS=$vte_cflags" >> $config_host_mak