some Makefile and hints tinkering

1. remove all window interface bits from compiler.370, and have
   the preceding include files set some variables to control
   the behavior of compiler.370 when it comes to c++.
2. some more common Makefile lines into sys/unix/hints/include/multiw-3.370.
3. make it so you can pass cppregex=1 on the Make command line to build with
   sys/share/cppregex.cpp instead of posixregex.c
4. fix sys/share/cppregex.cpp so that it will build with clang compiler
   (required an additional header include). I don't know if it would have
   worked with g++ without that change. The include can be placed into an #ifdef
   block if there's an issue with the change on other compilers.
5. Anything that needs to compile using c++ (Qt, sys/share/cppregex.cpp) can
   just ensure that CPLUSPLUS_NEEDED Makefile variable is set above the lines
   in compiler.370 to ensure that things get set up for c++. It no longer
   checks specifically for Qt. That is what sys/unix/hints/include/multiw-2.370
   does now.

sys/unix/hints/include/multiw-2.370

@@ -102,12 +102,71 @@ XTRASRC += tile.c
 XTRAOBJ += $(TARGETPFX)tile.o
 endif
 
+ifndef WANT_WIN_QT
+ifdef WANT_WIN_ALL
+WANT_WIN_QT=1
+endif
+ifdef WANT_WIN_QT5
+ifndef WANT_WIN_QT
+WANT_WIN_QT=1
+endif   # not WANT_WIN_QT
+endif   # WANT_WIN_QT5
+ifdef WANT_WIN_QT6
+ifndef WANT_WIN_QT
+WANT_WIN_QT=1
+endif   # not WANT_WIN_QT
+endif   # WANT_WIN_QT6
+endif   # not def WANT_WIN_QT
+
+ifdef WANT_WIN_QT
+# WANT_WIN_QT5 is the default
+ifndef WANT_WIN_QT5
+ifndef WANT_WIN_QT6
+WANT_WIN_QT5=1
+endif   # not WANT_WIN_QT6
+endif   # not WANT_WIN_QT5
+endif   # WANT_WIN_QT
+
 ifdef WANT_WIN_QT
 WINCFLAGS += -DQT_GRAPHICS
 WINSRC += $(WINQTSRC)
 WINOBJ0 += $(WINQTOBJ)
 XTRASRC += tile.c
 XTRAOBJ += $(TARGETPFX)tile.o
+#
+ifndef CPLUSPLUS_NEEDED
+CPLUSPLUS_NEEDED = 1
+endif   # CPLUSPLUS_NEEDED
+ifdef WANT_WIN_QT6
+CPLUSPLUS_NEED17 = 1
+CPLUSPLUS_NEED_DEPSUPPRESS = 1
+endif   # WANT_WIN_QT6
+endif   # WANT_WIN_QT
+
+ifeq "$(GIT)" "1"
+ifndef GITSUBMODULES
+GITSUBMODULES=1
+endif
+endif
+
+ifeq "$(git)" "1"
+ifndef GITSUBMODULES
+GITSUBMODULES=1
+endif
+endif
+
+ifeq "$(CPPREGEX)" "1"
+REGEXOBJ=$(TARGETPFX)cppregex.o
+ifndef CPLUSPLUS_NEEDED
+CPLUSPLUS_NEEDED = 1
+endif
+endif
+
+ifeq "$(cppregex)" "1"
+REGEXOBJ=$(TARGETPFX)cppregex.o
+ifndef CPLUSPLUS_NEEDED
+CPLUSPLUS_NEEDED = 1
+endif
 endif
 
 #end of hints/include/multiw-2.370