Check*: Refactor variable handling

Commit 5abaf65154 (Check*: Restore toleration of `;-W...` in
CMAKE_REQUIRED_FLAGS, 2026-06-24, v4.4.0-rc3~2^2) introduced logic to
prevent `-W*` arguments in `CMAKE_REQUIRED_FLAGS` from being passed as
CMake arguments. It was never the intent of `CMAKE_REQUIRED_FLAGS` to
support such usage; however, since older versions of CMake would
silently accept such arguments, some users were unintentionally passing
compiler warnings as CMake arguments. Although this usage is incorrect,
it was silently tolerated until CMake 4.4, which began treating unknown
`-W*` as an error.

Because it turns out that modules besides `CheckSource{Compiles,Runs}`
are affected, we are going to need the logic added by that commit in
many more places. As a first step toward accommodating that need, move
the logic to a new internal helper. While we're at it, try to generally
consolidate the common logic for handling the various variables that the
`Check*` family of modules uses for injecting arguments into the
ultimate `try_compile` calls.