modules: updated FindSDL2.cmake.
modules: updated FindSDL2.cmake.

Fixes building on OSX.

mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples