Fix 'ISO C90 does not support __func__' error with GCC 5+

[squeek502]

Travis now uses GCC 5 by default, so the -std=c89 -pedantic-errors job fails with errors like

../minunit.h:143:82: error: ISO C90 does not support ‘__func__’ predefined identifier [-Wpedantic]
 nunit_last_message, MINUNIT_MESSAGE_LEN, "%s failed:\n\t%s:%d: %s", __func__, _
                                                                     ^

• Using __extension__ __FUNCTION__ is what is recommended by https://gcc.gnu.org/gcc-5/porting_to.html
• __STDC_VERSION__ is not defined by GCC when -std=c89 is specified

[siu]

Thanks for the fix!