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

configure

@@ -2335,6 +2335,8 @@ elif enabled ccc; then
     add_cflags -msg_disable nomainieee
     add_cflags -msg_disable ptrmismatch1
     add_cflags -msg_disable unreachcode
+elif enabled gcc; then
+    check_cflags -fno-tree-vectorize
 fi
 
 # PIC flags for shared library objects where they are needed