change COMPILER_FAMILY_IS flags to CACHE variables so users can have …

[robinson96]

…control over detected family type.

[white238]

I am curious what caused this to not work?

[white238]

I am concerned these variables would be pushed from directory scoped to global and are not prefixed with "BLT_". Do codes ever use these variables? Can we just prefix them now so they dont potentially conflict with project variables? Thoughts @kennyweiss ?

Edit: RAJA does use these variables...

[kennyweiss]

I agree with @white238's concern -- it seems that if we're exposing these to the cache, we should prefix them with BLT_, (and give users some notice that we're doing this).

I am also interested in hearing about what prompted this. Being able to modify this might lead to some unexpected behavior, so it would be good to understand the use case.