fexecve doesn't exist on mac/ios

[Susurrus]

For some reason it's ignored in build.rs, but my /usr/include/unistd.h on Mac OS X 10.11.6 doesn't have it. It's actually nowhere in /usr/include/. Using this results in a linking error, so I'd suggest it be removed on those platforms even if it's technically a breaking change (as nobody can possibly be using it on those platforms).

I can file a PR to remove it, but what to do about build.rs, anything? Should I change it to be if mac for fexecve to make sure it never creeps back in accidentally?

[alexcrichton]

Seems fine to me yeah to remove! IIRC these are ignored b/c the exec-family of functions are really hard to verify (the signatures in C are tough for ctest to generate to match)