This adds GLUON_BUILDTYPE, which is an option that allows specifying an alternate
base configuration
Other additions:
- GLUON_PREFIX option to specify the filename prefix, if
VERSION_DIST is set
- Skip gluon target-definitions if TARGET_ALL_PROFILES is set
- Allow overriding any file in targets/ by adding the file to site/
- Allow loading the original file using include_gluon() from gluon
