Build the geom_uncompress(4) module by default.

Fix geom_uncompress(4) module loading. Don't link zlib.c (which is a module
itself) directly.

The built module was verified and used to read a few mkulzma(8) images on
amd64 to validate some of the informations on the manual page.

While here, don't overwrite CFLAGS.

Reviewed by: ray
Approved by: adrian (mentor)