[flang] Avoid dependency of runtime library on pthread for MinGW

When building the Fortran runtime on MinGW, `clock_gettime` is currently used. That function is provided by the `pthread` library on that platform. That means that all programs that link `libFortranRuntime` also require to be linked with `pthread` on that platform.

There is already a code path (for MSVC) that doesn't use `clock_gettime` in the implementation of the Fortran library.
Use the same code path also on MinGW by undefining `CLOCKID`.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D149051

flang/runtime/time-intrinsic.cpp

@@ -56,7 +56,12 @@ template <typename Unused = void> double GetCpuTime(fallback_implementation) {
   return -1.0;
 }
 
-#if defined CLOCK_PROCESS_CPUTIME_ID
+#if defined __MINGW32__
+// clock_gettime is implemented in the pthread library for MinGW.
+// Using it here would mean that all programs that link libFortranRuntime are
+// required to also link to pthread. Instead, don't use the function.
+#undef CLOCKID
+#elif defined CLOCK_PROCESS_CPUTIME_ID
 #define CLOCKID CLOCK_PROCESS_CPUTIME_ID
 #elif defined CLOCK_THREAD_CPUTIME_ID
 #define CLOCKID CLOCK_THREAD_CPUTIME_ID