Fix non-Linux UNIX-y builds

[Kreeblah]

malloc.h is a Linux-specific header, so without ifdef-ing it, it breaks Mac (and presumably BSD) builds.

[carstene1ns]

This header is shipped by glibc, but will be included from stdlib.h, so we can simply remove it (like in #100).

Going to close this one in favor of the other. Thank you!