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

[mmuetzel]

Summary

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.

[mmuetzel]

See also the discussion downstream here:
https://github.com/msys2/MINGW-packages/pull/16032#issuecomment-1460622045

And a similar patch that is applied for Flang in MSYS2:
https://github.com/msys2/MINGW-packages/blob/c12ef564342b37665182cfe8c1bbd60e9200aeff/mingw-w64-flang/0004-do-not-clock_gettime-on-mingw.patch

[vzakhari]

Thank you! LGTM

[mmuetzel]

Thank you for the review.
I updated the diff to fix the grammar in the comment. (Sorry for not noticing before.)

I don't have push privileges for the repository. Could you please push that diff for me?

[vzakhari]

Hi @mmuetzel, I will merge this today, but please obtain the commit access - it is easy ;)

flang/runtime/time-intrinsic.cpp

template <typename Unused = void> double GetCpuTime(fallback_implementation) {
   std::clock_t timestamp{std::clock()};
   if (timestamp != static_cast<std::clock_t>(-1)) {
     return static_cast<double>(timestamp) / CLOCKS_PER_SEC;
   }
   // Return some negative value to represent failure.
   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
 #elif defined CLOCK_MONOTONIC
 #define CLOCKID CLOCK_MONOTONIC
 #elif defined CLOCK_REALTIME
 #define CLOCKID CLOCK_REALTIME
 #else