[jamfiles] Refactor to share more between Darwin builds.
[jamfiles] Correctly deal with debug info on Mac OS X.

Mac OS X lazily links debug info. That is, it leaves it in the
various .o files until dsymutil is used to link the debug info
into a bundle. Doing this and then stripping our actual libraries
and executables should result in an improved out of the box experience.
Merge pull request #816 from waywardmonkeys/fix-darwin-debug-info

Fix darwin debug info