This is to support the GN build, which won't read the MAS_BUILD
environment variable, and will instead rely on the GN args system to
toggle between MAS/non-MAS.
This change defines the static_library GN targets only during the second pass
of the static_library build. The config targets had to be moved out of the main
`BUILD.gn` to prevent undefined variable errors because I removed the
`if (obj_something)` checks.
Jeremy Apthorp
deepak1556
Nitish Sakhawalkar
Ales Pergl