GNUSTEP_MAKEFILES needs to be set earlier

[atsampson]

When building without GNUSTEP_MAKEFILES already set in the environment, the configure script shipped in gnustep-back-0.31.0.tar.gz fails immediately with:

configure: error: cannot find required auxiliary files:

There is code to set GNUSTEP_MAKEFILES automatically, but it doesn't run until after the generated configure script has already tried to use it to find config.guess and config.sub; it probably needs moving earlier in the file.