[msvc] Fix MONO_INTERNAL undefined in Boehm
[msvc] Allow cygwin "make" to be called after winsetup.bat

As-is, winsetup.bat replaces config.h with winconfig.h, which is needed for the MSVC compilation - but breaks all further use of cygwin's make (e.g. to build the class library, or run "make install").

This patch introduces a new cygconfig.h as a backup prior to squashing config.h (if it exists, so the runtime can still be built without cygwin ever being used), and makes winconfig.h use it on non-MSVC compilers (i.e. cygwin)