compiling 64 bit libraries

[Gideon Simpson]

I currently have a perfectly good 32bit GSL library, but I'd like to have a 64 bit version so that I will more easily be able to link against it using MATLAB's MEX.  I'm not entirely sure how to go about doing this.  I have an OS X 10.6 machine with a native install of GCC 4.4.2.  Which flags do I want to use?  How do I then ensure it's installed in /usr/local/lib/x86_64?

-gideon