Modify valac --target-glib.

valac --ccode --target-glib=2.36 can disable g_type_init in the
generated C files since g_type_init is deprecated in glib 2.36.
But ibus tarball includes C files so that users do not require
vala and vala versions are different between dist and build.
If dist could have glib 2.36 to generate the ibus tarball but
a user build would have glib 2.34, the user build would be
failed because glib 2.34 requires g_type_init.
This change makes sure the target-glib is minimum of the glib
version for ibus.

Review URL: https://codereview.appspot.com/10156044