build: subproject improvements:

 - don't expose source in dependency to avoid name clashes
 - add babl path variables to dependency object
 - add babl path variables to uninstalled pc
2 jobs for master in 3 minutes and 52 seconds
Status Job ID Name Coverage
  Build
passed #1180667
latest-lcms

00:03:52

passed #1180668
latest-nolcms

00:03:51