Fix build on macOS Sierra

Fix build on macOS Sierra

Before this fix, the build died with `ar: no archive members specified
when linking the empty libreplacements.la.` because macOS Sierra does
not require the replacements anymore. With this fix, `ffs.c` and
`strtok_r.c` are always getting compiled (even when they are empty) to
prevent the error. Also removed the unused
`CVC4_NEEDS_REPLACEMENT_FUNCTIONS` from `configure.ac` and added an
`#ifndef HAVE_FFS` to `ffs.c` for consistency with `strtok_r.c`.