The glib_dynamic detection does not work because the dependency is
overridden in the main meson.build.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
[Rewritten commit message, added requirement in qga/meson.build - Paolo]
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>