Use a macro to redirect calling code from the official name to the
ff_ prefixed one.

Detecting these functions in configure can be tricky (on mingw, they
are conditionally available depending on posix feature defines).
If configure didn't detect them, but they still are visible at
compile time (due to an unrelated header defining the posix feature
defines), providing the local fallback versions with a prefixed
name is safer.
Signed-off-by: Martin Storsjö <martin@martin.st>