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 subproject in 5 minutes and 11 seconds
Status Job ID Name Coverage
  Build
passed #1179027
latest-lcms

00:05:06

passed #1179028
latest-nolcms

00:05:11