Remove use of UseObjectLibraries from Makefile and Ninja generators. It
is not needed now because those generators use GetExternalObjects
which already contains the objects from object libraries.
The VS10 generator calls both the UseObjectLibraries and the GetExternalObjects
methods. Ensure that duplicates are not created by skipping objects
from object libraries in handling of GetExternalObjects.
Similarly, fix VS6, VS7 and Xcode object handling by skipping
external objects from OBJECT_LIBRARY usage as appropriate.
The error message in the BadSourceExpression1 test is now reported
by the generator expression evaluator, so it has different text.
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.
Stephen Kelly
Brad King