Load libc on darwin in any cases

[pradal]

Per #888

load_dll('c').free fail on Darwin, without MacPort or Homebrew, because libc is not found on my system.
By default, on Mac, libc is provided by libSystem.dylib.

A robust solution is to replace load_dll('c').free by CDLL(None).free because Python is already using libc.

CDLL(None) internally calls dlopen(NULL), and as the dlopen
manpage says, "If filename is NULL, then the returned handle is for the
# main program". This way we can let the linker do the work to figure out
# which libc Python is actually using.

[coveralls]

Coverage remained the same at 81.717% when pulling 98f6b36 on pradal:libc_mac into 81ddee0 on Toblerity:maint-1.7.

[sgillies]

👍

[sgillies]

@pradal thank you!

[adamjstewart]

I just ran into this same issue on macOS. Shouldn't this be merged to master as well?