Previously the mscv Makefile.wnm didn't have rules to generate funcs.h
and help.c, meaning that it could only be used to build release
tarballs (which already have these files), but not git snapshots.

Now we have a new independent rule "generated" (not part of "all"),
which generates them. Because typical msvc setups do't have the tools
which are used elsewhere to generate these files, this commit adds
buildgen.c which, when compiled, can be used instead.

It's not part of "all" because when cross compiling, e.g. an arm
binary on Windows 64, buildgen.exe should be native and not arm,
so this allows making "generated" with a native toolchain, and then
"all" using the arm toolchain.

We already have buildgen.c which can be used to generate funcs.h and
help.c without grep/sed/perl/sh/etc, which can be useful in mingw
setups on Windows which don't bring the whole posix arsenal with them.

The default is still the posix tools, but now adding WINGEN=1 will
use buildgen.c instead of the posix tools.

The compiler which is used with buildgen.c is the same compiler used
at the rest of Makefile.wng, so it should be native (not cross build).