check to defend against bad linker flags?

observation:

If you add `CMAKE_EXE_LINKER_FLAGS` and have a typo (e.g: misspell a lib name), this will cause failures downstream when you get to BLT-builtin TPL support. In my case, finding MPI failed and there was no output that would signal it is related the lib flags, just "I can't find MPI!"

Since these flags are used everywhere, a compile check that uses these flags early could detect an issue and allow us to provide a better error message.

Start by tracing where CMAKE_EXE_LINKER_FLAGS are applied and where BLT-builtin TPL support performs MPI detection. Run the existing configuration or MPI checks with an intentionally invalid linker flag to reproduce the silent failure. Done means an early compile check reports the linker-flag problem distinctly before downstream MPI detection fails.

Written by the indexing model from the issue text.