Makefile tweaks to suppress va_list warnings

There's a bug in devkitPro's version of GCC that warns of PSABI issues
in system headers even though they aren't needed for them. This is fixed
in a newer version of GCC that devkitPro has not yet adopted.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42748
Merge pull request #67 from xdpirate/makefile-patch

Makefile tweaks to suppress va_list warnings