Update make.conf: Fix compilation under cygwin

Hi Ed,

The cygwin compilation failed with the latest changes in make.conf. GLUT_ON:=0 seems to cause the problem.

If you want to tag a new release, please tag pre-4.0 instead of 4.0 directly. This is required to reserver a DOI for the new version and then include it in its metadata of the actual release. 4.0 will then be tagged by the CI pipeline automatically.