crossplatform.mk: add cygwin detection for disabling windows shell tools and path...

crossplatform.mk: add cygwin detection for disabling windows shell tools and path separator hacks in order to allow building via git under windows. also allow path to make command that contains spaces.
 - i.e.: git rebase -i --exec "git clean -xdf && make ENABLE_SSL=y EDASQLiteCipher=y -j 16" HEAD~

 - 'cd <dir-name> && $(MAKE)' vs 'cd <dir-name> && $(_MAKE)' -- gnu make is no longer nice enough to do as if there was a + in front of the rule, need to add it ourselves