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.
The add_library(INTERFACE) and Interface Libraries documentation
list all the INTERFACE_* properties and target_* commands that
can be used to define the interface. Add INTERFACE_SOURCES
and target_sources() to these lists for completeness.
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.