win32/makefiles: only add -std:c++20 for USE_CPLUSPLUS

and note that only 2019 or later can build as C++, since the
use of designated initialisers means we need at least C++20 and
only 2019 and 2022 support that.

"cl /?" for 2019 doesn't list "-std:c++20" as an option at least of
19.29.30148, but it does accept it.

win32/GNUmakefile

@@ -180,6 +180,7 @@ DEFAULT_INC_EXCLUDES_DOT := define
 
 #
 # Uncomment this if you want to build everything in C++ mode
+# This requires gcc or at least MSVC 2019.
 #
 #USE_CPLUSPLUS	:= define
 
@@ -671,7 +672,10 @@ INCLUDES	= -I.\include -I. -I..
 #PCHFLAGS	= -Fpc:\temp\vcmoduls.pch -YX
 DEFINES		= -DWIN32 -D_CONSOLE -DNO_STRICT
 LOCDEFS		= -DPERLDLL -DPERL_CORE
-CXX_FLAG	= -TP -EHsc -std:c++20
+CXX_FLAG	= -TP -EHsc
+ifeq ($(USE_CPLUSPLUS),define)
+CXX_FLAG	+= -std:c++20
+endif
 EXTRACFLAGS	= -nologo -GF -W3
 
 ifeq ($(CCTYPE),MSVC120)

win32/Makefile

@@ -120,6 +120,7 @@ DEFAULT_INC_EXCLUDES_DOT = define
 
 #
 # Uncomment this if you want to build everything in C++ mode
+# This requires at least MSVC 2019.
 #
 #USE_CPLUSPLUS	= define
 
@@ -431,7 +432,10 @@ INCLUDES	= -I$(COREDIR) -I.\include -I. -I..
 #PCHFLAGS	= -Fpc:\temp\vcmoduls.pch -YX
 DEFINES		= -DWIN32 -D_CONSOLE -DNO_STRICT
 LOCDEFS		= -DPERLDLL -DPERL_CORE
-CXX_FLAG	= -TP -std:c++20 -EHsc
+CXX_FLAG	= -TP -EHsc
+!IF "$(USE_CPLUSPLUS)" == "define"
+CXX_FLAG	= $(CXX_FLAG) -std:c++20
+!ENDIF
 EXTRACFLAGS	= -nologo -GF -W3
 
 !IF "$(CCTYPE)" == "MSVC120"