LIBVIRT_ARG_WITH_ALT is more generic than LIBVIRT_ARG_WITH, which
is tailored at switching features on and off.

Rename the macros according to their intended purpose, and add
some documentation to help developers pick between the two.

Usage of AC_REQUIRE will mess with order how LIBVIRT_CHECK_* macros
are composed into configure.ac.  This ensures that the output of
configure --help is properly ordered and grouped into sections.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Following patch will update LIBVIRT_CHECK_(LIB|LIB_ALT|PKG) macros in
a way that you will be able to call a another macro as a fallback if
the first one fails.  To allow that, we need to move the
LIBVIRT_ARG_WITH out of those macro to not have two or more same lines
in output of "configure --help".
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

Most checks for libraries take the same format

  * --with-libFOO=yes|no|check|/some/path  argument
  * check for a function NNN in libFOO.so
  * check for a header file DDD/HHH.h
  * Define a WITH_FOO config.h symbol
  * Define a WITH_FOO make conditional
  * Substitute FOO_CFLAGS and FOO_LIBS make variables
  * Print CFLAGS & LIBS summary at the end

Doing all this correctly is rather difficult, typically
done by copy+paste of a previous usage. Further small
improvements people make are not applied to all previous
usages.

Improve this by creating some helper macros to apply
good practice. First, to perform the actual checks:

  LIBVIRT_CHECK_LIB([SELINUX], [selinux],
     [getfilecon], [selinux/selinux.h])

This checks for 'getfilecon' in -lselinux, and the
existence of 'selinux/selinux.h' header file. If successful
it sets SELINUX_CFLAGS and SELINUX_LIBS. The WITH_SELINUX
config.h macro and WITH_SELINUX make conditional are also
defined.

In some cases we need to check two variants of the same
library

  LIBVIRT_CHECK_LIB_ALT([SASL], [sasl2],
                        [sasl_client_init], [sasl/sasl.h],
                        [SASL1], [sasl],
                        [sasl_client_init], [sasl/sasl.h])

This checks for sasl_client_init in libsasl2, and if that
is not found, checks sasl_client_init in libsasl. If the
first check succeeds WITH_SASL is set, while if the second
check succeeds *both* WITH_SASL and WITH_SASL1 are set.

If the library supports pkg-config, then another variant
is available

  LIBVIRT_CHECK_PKG([AVAHI], [avahi-client], [0.6.0])

This checks for avahi-client >= 0.6.0 via pkg-config
and sets WITH_AVAHI if found.

Finally to print a summary of CFLAGS & LIBs found (if any):

  LIBVIRT_RESULT_LIB([SELINUX])
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>