Change how win32/common/config.h is updated

Change how win32/common/config.h is updated

Generate win32/common/config.h-new directly from config.h.in,
using shell variables in configure and some hard-coded information.
Change top-level makefile so that 'make win32-update' copies the
generated file to win32/common/config.h, which we keep in source
control.  It's kept in source control so that the git tree is
buildable from VS.

This change is similar to the one recently applied to GStreamer,
except that it adds a few -base specific defines.