Use -std=gnu99 in tools/llvm-c-test/CMakeLists.txt

With a current mingw (gcc 4.8.1) it looks like we hit some variation of

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40278

The end result is that off_t is not defined and the build fails without this
patch.

llvm-svn: 198749