--with-gcc: export the CC environment variable, so the setting gets
picked up by sub-configures.  Perhaps we should be advising people to use

  CC=c:/mingw/bin/gcc ./configure

because that works for all configure scripts, not just the top level one.

Background:

 - We want --with-gcc to do the right thing, because we don't want
   to require Cygwin users to put c:/mingw/bin first on their
   path: that would break their Cygwin environment.

 - The build system should work with *no* gcc on your PATH, as long
   as you use --with-gcc (equivalently, CC=... ./configure).