Reenable checks for unique compilers

The travis CI build fails even with this.  I kind of hate Travis CI becuase
when something goes wrong, it is very hard to debug because you can't log onto
the VM to address the problem.
Remove duplication.
Refactor to get ready to test CXXFLAGS and CMAKE_CXX_FLAGS
Add behavior for CXXFLAGS/CMAKE_CXX_FLAGS and skip all when USE_XSDK_DEFAULTS=OFF

Now the XSDKDefaults.cmake module does nothing when USE_XSDK_DEFAULTS=OFF.