Fix build on OS X

On OS X the malloc header is inside /usr/include/malloc. Add that to the
include paths instead of changing the code to conditionally include
`malloc/malloc.h`.

uname_S is taken from git's Makefile [1]

  [1]: http://git.kernel.org/cgit/git/git.git/tree/Makefile
Ignore .dSYM files

These are debug symbols created when compiling on OS X.
Merge pull request #121 from pigoz/fix-build-macosx

OS X fixes