cmTarget: Fix listing of source files at configure-time.

Since commit e5da9e51 (cmTarget: Allow any generator expression in
SOURCES property., 2014-03-18), source files are computed by
true evaluation of generator expressions, including TARGET_OBJECTS.
This evaluation requires the presence of cmGeneratorTarget objects
since commit bf98cc25 (Genex: Evaluate TARGET_OBJECTS as a normal
expression., 2014-02-26).

Ensure that we don't attempt to evaluate the TARGET_OBJECTS generator
expression at configure-time, as can happen if CMP0024 or CMP0026
are OLD.  Use old-style parsing of the source item to extract
object target names in that case.

Avoid calling GetProperty("SOURCES") to bypass warnings from CMP0051.
Refactor existing logic in GetLanguages which is similar in intent to
the new GetSourceFiles code.
Merge topic 'fix-SourceFiles-configure-time' into next

b8af2011 cmTarget: Fix listing of source files at configure-time.