tell qmake to use gcc, g++

because otherwise it defaults to clang on mac os x.
However to use the boost installed by, e.g., macports, requires explicitly adding to `Tool.pro` the include path to the macports headers (`-I/opt/local/include`). This is not platform agnostic.

Even if `boost` is included, `clang` cannot `#include <unordered_set>` as required by `src/BFAbstractionLibrary/BFCuddMintermEnumerator.h`. The include statement need to be replaced by `#include <tr1/unordered_set>` with any other relevant changes, but this leads to further errors due to `clang` incompatibilities.

Also, the option: `-std=gnu++0x` used in `Tool.pro` is available only for `gcc`, not `clang`.
prevent qmake from appending -Xarch_x86_64

because gcc and g++ do not have this option, only clang and clang++.
note in readme that gcc and g++ are required on OS X
Merge pull request #4 from johnyf/master

use gcc, g++ instead of clang on OS X