Make feature detection values internal

Previously, the "feature detection options" (STACK_DETAILS_* and STACK_WALKING_*) had to be manually set by an user, but now, we (attempt) to detect libraries based on find_library, thus we don't need those to be cache values anymore. Libraries in non-default paths can be manually added in the corresponding _PATH cache value and for testing purposes, users can also disable libraries by supplying an empty string.