glib2 rbgobj_typeinterface.c: drop GLIB_CHECK_VERSION < 2.12
glib2 rbglib_bookmarkfile.c: drop GLIB_CHECK_VERSION < 2.12
glib2 rbglib_keyfile.c: drop GLIB_CHECK_VERSION < 2.12
glib2 rbglib_unichar.c: drop GLIB_CHECK_VERSION < 2.12
glib2 rbglib_maincontent.c: drop GLIB_CHECK_VERSION < 2.12
glib2 rbglib_source.c: drop GLIB_CHECK_VERSION < 2.12
add required libglib version