fortran vs cmake vs compiler flags

BLT `blt_add_target_compile_flags` is using `target_compile_options` - if you pass flags as a cmake string, the full string gets quoted before passed to the fortran compiler. This confuses `gfortran` if you have more than one option.

If these are passed as a cmake list, things make it to the fortran compiler correctly.

I am using `blt_append_custom_compiler_flag`, which does not seem to be creating a list.

(I am using an older blt, so maybe this has been fixed?)

Start with BLT's blt_add_target_compile_flags and blt_append_custom_compiler_flag entry points, then reproduce the behavior using CMake and gfortran with multiple compiler options. Confirm that flags passed as a CMake string are split correctly before reaching the Fortran compiler, while preserving correct handling of CMake lists.

Written by the indexing model from the issue text.