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.
Add "RunCMake.ObjectLibrary" test to verify that unsupported use cases
are rejected with errors:
* An OBJECT library may not reference another object library
* An OBJECT library may not be referenced in target_link_libraries
* An OBJECT library may not contain non-compiling sources
* An OBJECT library may not have pre/post build/link commands
* An OBJECT library may not be installed, exported, or imported
Also verify that invalid $<TARGET_OBJECTS:...> expressions are
diagnosed.
Brad King