cmTarget: Don't assert on object libraries for configure-time location.

Commit b8af2011 (cmTarget: Fix listing of source files at
configure-time., 2014-04-13) refactored a GetObjectLibrariesCMP0026
method out of GetLanguages.  In flight, a conditional use of a target
if available was changed to an assert-available.

This code is only used to read the LOCATION property at configure
time, when the link information is incomplete, and not all targets
are defined, so the assert is inappropriate, even though it can lead
to incorrect information being generated.  CMP0026 warns about the
potentially incorrect information anyway.
Merge topic 'no-assert-missing-objlib' into next

d78ec426 cmTarget: Don't assert on object libraries for configure-time location.