apparently there is a macro

I'll squash this back into the

  win32: detect ucrt versions of gcc and allow building with it

once it passes CI

win32/GNUmakefile

@@ -487,15 +487,6 @@ GCCVER2		:= $(shell for /f "delims=. tokens=1,2,3" %%i in ('$(GCCBIN) -dumpversi
 GCCVER3		:= $(shell for /f "delims=. tokens=1,2,3" %%i in ('$(GCCBIN) -dumpversion') do echo %%k)
 endif
 
-# are we using UCRT or MSVCRT?
-ifeq ($(CCTYPE),GCC)
-CRT_TYPE := $(shell gcc --version 2>&1 | find "msvcrt" >nul && ( echo msvcrt) || ( echo ucrt) )
-else ifeq ($(CCTYPE),MSVC120)
-CRT_TYPE := msvcrt
-else
-CRT_TYPE := ucrt
-endif
-
 # Set the install location of the compiler headers/libraries.
 # These are saved into $Config{incpath} and $Config{libpth}.
 ifneq ($(GCCCROSS),)
@@ -786,11 +777,6 @@ TESTPREPGCC	=
 
 endif
 
-ifeq ($(CRT_TYPE),ucrt)
-BUILDOPT	+= -DPERL_WIN32_UCRT
-MINIBUILDOPT	+= -DPERL_WIN32_UCRT
-endif
-
 CFLAGS_O	= $(CFLAGS) $(BUILDOPT)
 
 RSC_FLAGS	=
@@ -1164,8 +1150,7 @@ ifeq ($(CCTYPE),GCC)
 	echo # WIN64=$(WIN64)&& \
 	echo # ARCHITECTURE=$(ARCHITECTURE)&& \
 	echo # ARCHNAME=$(ARCHNAME)&& \
-	echo # MAKE=$(PLMAKE) && \
-	echo # CRT_TYPE=$(CRT_TYPE)
+	echo # MAKE=$(PLMAKE)
 else
 	@echo # CCTYPE=$(CCTYPE)&& \
 	echo # WIN64=$(WIN64)&& \

win32/Makefile

@@ -442,7 +442,6 @@ EXTRACFLAGS	= -nologo -GF -W3
 LIBC		= msvcrt.lib
 !ELSE
 LIBC		= ucrt.lib
-DEFINES		= $(DEFINES) -DPERL_WIN32_UCRT
 !ENDIF
 
 !IF  "$(CFG)" == "Debug"

win32/win32.h

@@ -262,7 +262,7 @@ extern const __declspec(selectany) union PerlNan __PL_nan_u = { 0x7FF80000000000
 #endif /* ifdef _MSC_VER */
 
 /* The CRT was rewritten in VS2015. */
-#ifdef PERL_WIN32_UCRT
+#ifdef _UCRT
 
 /* No longer declared in stdio.h */
 EXTERN_C char *gets(char* buffer);
@@ -297,7 +297,7 @@ typedef struct
 #define PERLIO_FILE_flag(f) ((int)(((__crt_stdio_stream_data*)(f))->_flags))
 #define PERLIO_FILE_file(f) (*(int*)(&((__crt_stdio_stream_data*)(f))->_file))
 
-#else /* ifdef PERL_WIN32_UCRT */
+#else /* ifdef _UCRT */
 
 /* Note: PERLIO_FILE_ptr/base/cnt are not actually used for GCC or <VS2015
  * since FILE_ptr/base/cnt do the same thing anyway but it doesn't hurt to