tools/configure.ac: add version check for glib2

tools/configure.ac: add version check for glib2

xen-unstable fails to build in a SLES10SP4 environment since a long time
because the included version of glib is slightly older than the required
glib version. According to the glib docs version 2.12 includes base64
support, but SLES10 is shipped with glib 2.8.6:

qemu-timer-common.o: In function `init_get_clock':
/usr/src/packages/BUILD/xen-4.2.25432/non-dbg/tools/qemu-xen-dir/qemu-timer-common.c:57:
undefined reference to `clock_gettime'
qga/guest-agent-commands.o: In function `qmp_guest_file_write':
qga/guest-agent-commands.c:249: undefined reference to `g_base64_decode'
qga/guest-agent-commands.o: In function `qmp_guest_file_read':
qga/guest-agent-commands.c:224: undefined reference to `g_base64_encode'
collect2: ld returned 1 exit status
make[3]: *** [qemu-ga] Error 1

Add a version check to toplevel configure to require at least glib 2.12.
This makes sure configure can detect the condition early instead of
failing later in the middle of tools build when qemu-upstream errors
out.

Please rerun autoconf after applying this.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>