Tests: Resolve TODO comment left in ObjectLibrary test

We now have a generic way to add -fPIC without explicitly modifying
CMAKE_C_FLAGS, so use it.
Tests: Fix broken dependency in ObjectLibrary test

The add_custom_command DEPENDS option needs a full path.
add_library: Allow arbitrary non-linked sources in OBJECT libraries

Loosen this restriction on OBJECT libraries to allow source files of any
name to be generated by custom commands or listed for reference in IDE
projects so long as they would not affect linking of a normal library.
Update the rejection message to be more specific about the looser
restriction.

Extend the ObjectLibrary test to cover a ".cmake" file generated by a
custom command in an OBJECT library.
Merge topic 'allow-OBJECT-library-extra-sources' into next

ce04e9ba add_library: Allow arbitrary non-linked sources in OBJECT libraries
de4f3e2c Tests: Fix broken dependency in ObjectLibrary test
54bf5858 Tests: Resolve TODO comment left in ObjectLibrary test