gbuild: one command for C/C++ objects

Change-Id: I835bd56123acfbfb53e0d3d86e5d66b929b3181f
gbuild: deduplicate by new gb_Helper_replace_if_different_and_touch

Change-Id: I98767944a74c2b6d90425e0232c40e20c8a6885a
Add config_libraries.h definitions to get library names easily.

Works for libmerged too, so we can get rid of LIBO_MERGELIBS.
And remove few more #defines in vcl.

It's generated by gbuild, so probably abusing config_host directory.
Open to improvements.

Change-Id: I87ab109bf109e42751766011daf076e9cdf8f5ee