Update GLIB_CHECK_COMPILE_WARNINGS macro to work with Autoconf 2.64

Quoting its NEWS file: "The m4sh macros AS_IF and AS_CASE can now be
used in shell lists.  The responsibility for supplying a trailing
newline now belongs to the call site, but since most users did not add
dnl, this generally results in fewer empty lines in configure."

http://bugzilla.gnome.org/show_bug.cgi?id=591840