CMake newer version

[hankim]

Hi,

I was trying to compile s2 python binding and had to update CMakefile a bit as I'm using newer version of CMake and some of directives are deprecated:

https://gist.github.com/hankim/d058ba1f26d9efd5aba296d6fa28e2e3

[hankim]

btw, I have very little knowledge about CMake, so someone may find a better way of handling this.

[hankim]

another issue is the dynamic library linking:
@rpath/libs2.dylib (compatibility version 0.0.0, current version 0.0.0)
and I had to manually change to:
/usr/local/lib/libs2.dylib (compatibility version 0.0.0, current version 0.0.0)
with the following command in Mac OS X:
sudo install_name_tool -change @rpath/libs2.dylib /usr/local/lib/libs2.dylib /usr/local/share/python/_pywraps2.so

I couldn't figure out where the link option is coming from.