configure: sanity check the glib library that pkg-config finds

Developers on 64-bit machines will often try to perform a
32-bit build of QEMU by running

  ./configure --extra-cflags="-m32"

Unfortunately if PKG_CONFIG_LIBDIR is not set to point to
the location of the 32-bit pkg-config files, then configure
will silently pick up the 64-bit pkg-config files and still
succeed.

This causes a problem for glib because it means QEMU will
be pulling in /usr/lib64/glib-2.0/include/glibconfig.h
instead of /usr/lib/glib-2.0/include/glibconfig.h

This causes problems because the 'gsize' type (defined as
'unsigned long') will no longer be fully compatible with
the 'size_t' type (defined as 'unsigned int'). Although
both are the same size, the compiler refuses to allow
casts from 'unsigned long *' to 'unsigned int *' as they
are different pointer types. This results in non-obvious
compiler errors when building QEMU eg

qga/commands-posix.c: In function â€˜qmp_guest_set_user_passwordâ€™:
qga/commands-posix.c:1912:55: error: passing argument 2 of â€˜g_base64_decodeâ€™ from incompatible pointer type [-Werror=incompatible-pointer-types]
     rawpasswddata = (char *)g_base64_decode(password, &rawpasswdlen);
                                                            ^
In file included from /usr/include/glib-2.0/glib.h:35:0,
                 from qga/commands-posix.c:14:
/usr/include/glib-2.0/glib/gbase64.h:52:9: note: expected â€˜gsize * {aka long unsigned int *}â€™ but argument is of type â€˜size_t * {aka unsigned int *}â€™
 guchar *g_base64_decode         (const gchar  *text,
         ^
cc1: all warnings being treated as errors

To detect this problem, add a check to configure that
verifies that GLIB_SIZEOF_SIZE_T matches sizeof(size_t).
If this fails print a warning suggesting that the dev
probably needs to set PKG_CONFIG_LIBDIR.

On Fedora x86_64 it passes with any of:

 # ./configure
 # PKG_CONFIG_LIBDIR=/usr/lib/pkgconfig ./configure --extra-cflags="-m32"
 # PKG_CONFIG_LIBDIR=/usr/lib64/pkgconfig ./configure --extra-cflags="-m64"

And fails with a mis-match

 # PKG_CONFIG_LIBDIR=/usr/lib64/pkgconfig ./configure --extra-cflags="-m32"
 # PKG_CONFIG_LIBDIR=/usr/lib/pkgconfig ./configure --extra-cflags="-m64"

ERROR: sizeof(size_t) doesn't match GLIB_SIZEOF_SIZE_T.
       You probably need to set PKG_CONFIG_LIBDIR
       to point to the right pkg-config files for your
       build target

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1453885245-15562-1-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
