Make TriBITS senstivie to XSDK_ENABLE_<LANG> instead of just setting it based on ${PROJECT_NAME}_ENABLE_<LANG>

[bartlettroscoe]

Currently, TriBITS just sets:

    SET(XSDK_ENABLE_C  ${${PROJECT_NAME}_ENABLE_C})
    SET(XSDK_ENABLE_CXX  ${${PROJECT_NAME}_ENABLE_CXX})
    SET(XSDK_ENABLE_Fortran  ${${PROJECT_NAME}_ENABLE_Fortran})

before it includes XSDKDefaults.cmake. However, it seems that from this discussion and it is implied in this specification, that users expect to be able to set XSDK_ENABLE_<LANG>=[ON|OFF] at configure time and have that respected.

But if we want TriBITS to be sensitive to the value XSDK_ENABLE_<LANG> set by the user (and not just overwrite it), then we have a problem. The issue is that if XSDK_ENABLE_<LANG> and ${PROJECT_NAME}_ENABLE_<LANG> are both set, but disagree, then what should happen? (This is a problem with duplicated variables in CMake. These are just global variables that can be set in a variety of ways in a variety of places and it is hard to address inconsistencies like this. So I might need some advice on the best way to handle this.)

Note that the general policy in TriBITS is that when an enable and a disable are inconsistent, then disables trump enables. If we follow this policy in this case, then if XSDK_ENABLE_<LANG>=OFF or ${PROJECT_NAME}_ENABLE_<LANG>=OFF are set, then <LANG> will be disabled (and if the other is ON, then a NOTE will be printed stating what happened.) The downside is that if someone sets XSDK_ENABLE_<LANG>=ON, but somewhere else Trilinos_ENABLE_<LANG>=OFF is set, then <LANG> will be off. That might make some people crazy. But if they just grep the CMake STDOUT they will see that XSDK_ENABLE_<LANG> got set to false and why.