We noticed that some feature-test macros were not conditional on
configuration flags like _LIBCPP_HAS_NO_FILESYSTEM. As a result, code
attempting to use FTMs would not work as intended.
This patch adds conditionals for a few feature-test macros, but more
issues may exist.
rdar://122020466
(cherry picked from commit f2c84211d2834c73ff874389c6bb47b1c76d391a)