Identify default mingw runtime (mingw builds only).

win32/configure/rt.c

@@ -0,0 +1,15 @@
+/*
+ Print the default runtime. $Config{libc}
+ will be set to this specified value.
+*/
+#include <stdio.h>
+#include <stddef.h>
+
+int main(void) {
+#if defined(_UCRT)
+  printf("-lucrt\n");
+#else
+  printf("-lmsvcrt\n");
+#endif
+  return 0;
+}