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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198749 91177308-0d34-0410-b5e6-96231b3b80d8