Fixes for broken compilation of mplayer on Mac OS X > 10.6 as well as an updated download strategy so that HEAD snapshots have revision numbers instead of "UNKNOWN"

The upstream HEAD version of configure fails to build on x86_64/Darwin for recent versions of clang. The fixes include:
- using -Wl,-pie to fix linking at the end
- using -DPIC with YASM to allow ffmpeg to compile properly
- --mdynamic-no-pic breaks compilation due to security features in newer versions of Mac OS X
- Overloading the SVN strategy to have a snapshot version written so that mplayer doesn't show up without a version number.