Merge pull request #1 from Vproject/buildfix

Fix build on some systems.
Initial YUV4MPEG2 output support.
speed up computing the SAD scores during motion estimation using SSE2 intrinsics (on this netbook with an AMD E-450, encoding sign_irene is no 11.05 seconds, instead of 13.2 seconds). This is hidden behind a #ifdef SSE2, so add "-D SSE2" to your compile options if you want to use this.
More flexible framerate.

YUV4MPEG2 output framerate can now be set with -f option.
-f option now supports fractional framerates as a ratio A:B, a fraction
C/D or decimal E.F
Use same variable to signal y4m for in- and output
Fix default y4m output filename extension.
Various y4m fixes.

* Seek to StartFrame if specified.

* Stop encoding when End Of File is reached.

* Use more standard single dash for stdin.
Merge pull request #2 from Vproject/y4moutput

Y4moutput
Try another buildfix.