Drop support for Mac OS X 10.5 (Leopard)

As of 5/3/2010, my survey results indicate that of approximately 19 represented users, only 2 say they are unwilling and/or unable to use a 10.6-only MWorks. (Both of those 2 responses come from the DiCarlo lab.) Note that this group of 19 does not include responses from the Cox lab (where all users already run 10.6) or Rust lab (where Nicole has said previously that she's willing to switch to 10.6).

Thus, since an overwhelming majority of users would be happy with a 10.6-only MWorks, we should drop support for 10.5. This doesn't involve much other than dropping the "Development (10.5 Compatible)" configs from Xcode projects and updating the buildbot config to build the "Development" target. Also, we can stop installing third-party libs in Developer/gcc40/{bin,include,lib} and just use Developer/{bin,include,lib}.