gbuild: More automagic for libmerged

Only gb_Library_use_libraries stays because I think it's a good idea to see
dependent libraries (potential candidates for merging) and hopefully the
list does not change often anyway.
This commit extends b6be3d3db3ffbc4c1a4528a8dc17aa95084e12ca

Change-Id: I2682456f53cb2e8d7ea63eae15f8979a3c828401