Add check/warnings for case-sensitive Fortran/FORTRAN variables

[kennyweiss]

CMake uses the mixed-case spelling Fortran for fortran-related variables, such as CMAKE_Fortran_FLAGS. For self-consistency, BLT uses all upper case spellings, such as BLT_FORTRAN_FLAGS.

Should we add some checks around the common fortran-related variables?

E.g. here: https://github.com/LLNL/blt/blob/88909956fa8cace238e0e975eeb444c157da0ffd/cmake/SetupCompilerOptions.cmake#L141-L147 and https://github.com/LLNL/blt/blob/88909956fa8cace238e0e975eeb444c157da0ffd/cmake/SetupCompilerOptions.cmake#L206-L211

This issue came up when working with a user code, where the capitalization issue was investigated as a possible error source.