Mplayer head clang and version fixes

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.