m4macros: Increment serial number of glib-2.0.m4
aclocal's ability to compare the version of macros and use the latest version relies on the serial number being incremented on every change, or at least on every functional change.
Fixes: 6f26637e "m4macros: replace obsolete macros AC_TRY_RUN and AC_TRY_LINK in glib-2.0.m4"