contrib: Fix the check for _UCRT

_UCRT is not a compiler internal define. Some build scripts may be
invoking the compiler with an explicit -D_UCRT, but if not, the
toolchain default is set by the toolchain headers. Therefore, include
crtdefs.h to pull in default headers for checking whether it is set
or not.

Also check for older forms of mingw-w64, which didn't use the
define _UCRT to signal the type of CRT.

contrib/src/main.mak

@@ -211,7 +211,14 @@ BUILDLDFLAGS ?= $(BUILDCFLAGS)
 # Do not export variables above! Use HOSTVARS or BUILDVARS.
 
 ifdef HAVE_WIN32
-ifneq ($(call cppcheck, _UCRT),)
+define UCRT_HEADER_CHECK :=
+#include <crtdefs.h> \n
+#if defined(_UCRT) || (__MSVCRT_VERSION__ >= 0x1400) || (__MSVCRT_VERSION__ >= 0xE00 && __MSVCRT_VERSION__ < 0x1000) \n
+# undef _UCRT \n
+# define _UCRT \n
+#endif \n
+endef
+ifneq ($(call cppcheck, _UCRT, $(UCRT_HEADER_CHECK)),)
 HAVE_UCRT = 1
 endif
 endif