Disable gcc auto-vectorisation

The auto-vectoriser in gcc is enabled by default at -O3 since version
4.3.  However, it provides no speed benefit, but does produce incorrect
code on many targets.  Disabling it for gcc should give more reliable
builds.

If the adventurous want it back, they can edit the makefile themselves.

Originally committed as revision 19439 to svn://svn.ffmpeg.org/ffmpeg/trunk