Applied patch to allow passing stdlib scons flag

This patch allows two new scons flags, with which you can compile for various of OS X build targets and C++ library versions.

On a OS X Mavericks 10.9 with freshly compiled homebrew libraries, you should now run with:

export QTDIR=/usr/local/Cellar/qt/HEAD
scons stdlib=libc++ verbose=1

This sets QTDIR explicitly to homebrew's Qt4 port (until QtTouchEvent issues are fixed for Qt5) and uses clang libc++ when linking: all current homebrew packages are linked with libc++ not libstdc++.

Please note that testing this on master right now fails, because of two linux specific commits which break compile on OS/X: I tested compile with 699232d578b170d92f21fb17e5f1a2e99786fb68 and it works now fine.