Set the minimum required version of CMake high enough to avoid the
warning for CMAKE_LEGACY_CYGWIN_WIN32. The warning appears on stderr
and breaks the expected output matching.
Add "RunCMake.ObjectLibrary" test to verify that unsupported use cases
are rejected with errors:
* An OBJECT library may not reference another object library
* An OBJECT library may not be referenced in target_link_libraries
* An OBJECT library may not contain non-compiling sources
* An OBJECT library may not have pre/post build/link commands
* An OBJECT library may not be installed, exported, or imported
Also verify that invalid $<TARGET_OBJECTS:...> expressions are
diagnosed.
Brad King