Use timespec_get if compiling with cl on Windows

src/time_now_stubs.c

@@ -21,6 +21,8 @@ CAMLprim value time_now_nanoseconds_since_unix_epoch_or_zero()
 
 #else
 
+#ifndef _MSC_VER
+
 #include <sys/types.h>
 #include <sys/time.h>
 
@@ -33,4 +35,36 @@ CAMLprim value time_now_nanoseconds_since_unix_epoch_or_zero()
     return caml_alloc_int63(NANOS_PER_SECOND * (uint64_t)tp.tv_sec + (uint64_t)tp.tv_usec * 1000);
 }
 
+#else /* compiling with cl */
+
+#if _MSC_VER < 1900 /* timespec_get is supported starting VS 2015 */
+
+/*
+ * Note that there are fallback implementations of gettimeofday for Windows.
+ * E.g., see https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/port/gettimeofday.c;h=75a91993b74414c0a1c13a2a09ce739cb8aa8a08;hb=HEAD
+ * I am not convinced there is much value in supporting Visual Studio versions
+ * older than 2015 in an OCaml library, so we just croak here instead of
+ * falling back.
+ */
+
+#error timespec_get is only available in Visual Studio 2015 (14.0) and later
+#endif
+
+#include <time.h>
+
+/*
+ * https://en.cppreference.com/w/c/chrono/timespec_get
+ * https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/timespec-get-timespec32-get-timespec64-get1?view=msvc-160
+ */
+
+CAMLprim value time_now_nanoseconds_since_unix_epoch_or_zero()
+{
+    struct timespec ts;
+
+    if (timespec_get(&ts, TIME_UTC) != TIME_UTC)
+        return caml_alloc_int63(0);
+    else
+        return caml_alloc_int63(NANOS_PER_SECOND * (uint64_t)ts.tv_sec + (uint64_t)ts.tv_nsec);
+}
+#endif
 #endif