gbuild: Do not require gb_Library_use_externals for libmerged

Instead, automatically get the externals when parsing other makefiles.
This commit fixes the build and hopefully avoids problems in future
because people didn't know they had to add externals for libmerged too.
Similar should be done for other functions like add_libs...

Change-Id: I5aea4c43f4b6838092ada81c310ef1100f241108
(cherry picked from commit b6be3d3db3ffbc4c1a4528a8dc17aa95084e12ca)
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

(cherry picked from commit 8cc1e7a7c3ec897129cc6318b254e8dafc682435)

Change-Id: I2682456f53cb2e8d7ea63eae15f8979a3c828401