More compile options for MacOS X.
More compile options for MacOS X.

Based on a patch from Dave Coombs.  I changed it to auto-detect the OS
platform, so I might have broken it, however.

pombredanne/bup,mhoeher/bup,gevaerts/bup,apenwarr/bup,gevaerts/bup,ToxicFrog/bup,tjanez/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,ToxicFrog/bup,gevaerts/bup,pombredanne/bup,mhoeher/bup,apenwarr/bup,mhoeher/bup,jbaber/bup,jbaber/bup,apenwarr/bup,apenwarr/bup,ToxicFrog/bup,tjanez/bup,pombredanne/bup,gevaerts/bup,tjanez/bup,jbaber/bup,jbaber/bup,tjanez/bup,ToxicFrog/bup,tjanez/bup,pombredanne/bup,mhoeher/bup,mhoeher/bup,pombredanne/bup,gevaerts/bup,jbaber/bup,jbaber/bup,ToxicFrog/bup