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

llvm/tools/llvm-c-test/CMakeLists.txt

@@ -8,7 +8,7 @@ set(LLVM_LINK_COMPONENTS
   )
 
 if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99 -Wstrict-prototypes")
+  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu99 -Wstrict-prototypes")
 endif ()
 
 add_llvm_tool(llvm-c-test