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.