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.
Showing with 92 additions and 21 deletions