implement USE_CPLUSPLUS build option in win32/GNUmakefile

implement USE_CPLUSPLUS build option in win32/GNUmakefile

win32/makefile.mk has had a C++ build option for a long time, but
GNUmakefile didn't. Over time, GNUmakefile and makefile.mk got more and
more syncronized in their features. There is very little left to implement
USE_CPLUSPLUS in GNUmakefile, so just implement it finally. This allows
the gmake/gcc 3.4.5 bundled with strawberry perl 5.8.9 to be used to build
USE_CPLUSPLUS mode perl since the dmake bundled with 5.8.9 whose version is
"Version 4.8-20070327-SHAY (Windows / MS Visual C++)"
has a bug generating cmd lines passed to cmd.exe which causes EUMM
makefiles to fail with

'..\..\miniperl.exe" "-I..\..\lib' is not recognized as an internal or
external command, operable program or batch file.

but newer dmakes such as "Version 4.12 (Windows / MinGW)" dont have this
bug. Researching the bug/regression with EUMM and ancient dmakes is left
for the future.