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