Use C99 printf statements in mingw

Change mingw build to use C99 printf statements. Otherwise, format specifiers such as PRId64 will not work on XP or earlier (see http://sourceforge.net/apps/trac/mingw-w64/wiki/gnu%20printf)

This bug was introduced with 51ed9ec9. 

Continued from #3237 (Silence inttypes.h compiler warnings on Windows) and #3236 (massive compiler warning spam).

This also adds strprintf() testcases so that hopefully this problem will be caught by the pulltester next time.