Fix redefinition of clock_gettime and clock_getres

winpthreads-git 12.0.0.r720 provides `clock_gettime` and
`clock_getres` as inline functions.

Author: nobu

include/ruby/win32.h

@@ -126,8 +126,15 @@ typedef unsigned int uintptr_t;
 #define O_SHARE_DELETE 0x20000000 /* for rb_w32_open(), rb_w32_wopen() */
 
 typedef int clockid_t;
+#if defined(__MINGW32__)
+#undef CLOCK_PROCESS_CPUTIME_ID
+#undef CLOCK_THREAD_CPUTIME_ID
+#undef CLOCK_REALTIME_COARSE
+#endif
+#if defined(HAVE_CLOCK_GETTIME) && !defined(CLOCK_REALTIME)
 #define CLOCK_REALTIME  0
 #define CLOCK_MONOTONIC 1
+#endif
 
 #undef utime
 #undef lseek

win32/win32.c

@@ -4761,6 +4761,7 @@ gettimeofday(struct timeval *tv, struct timezone *tz)
     return 0;
 }
 
+#if !defined(__MINGW32__) || !defined(HAVE_CLOCK_GETTIME)
 /* License: Ruby's */
 int
 clock_gettime(clockid_t clock_id, struct timespec *sp)
@@ -4800,7 +4801,9 @@ clock_gettime(clockid_t clock_id, struct timespec *sp)
         return -1;
     }
 }
+#endif
 
+#if !defined(__MINGW32__) || !defined(HAVE_CLOCK_GETRES)
 /* License: Ruby's */
 int
 clock_getres(clockid_t clock_id, struct timespec *sp)
@@ -4828,6 +4831,7 @@ clock_getres(clockid_t clock_id, struct timespec *sp)
         return -1;
     }
 }
+#endif
 
 /* License: Ruby's */
 static char *