Fix build of lame with GCC 4.9.

It seems GCC 4.9 got way pickier when compiling this SSE stuff in Lame. I tried to make the requirement of SSE as small in scope as possible (only the affected source file), so lame still runs on older x86 hardware (SSE2 is from ~2001...) - but couldn't test it since I don't own hardware that old anymore ;)

I had to manually disable GTK in the configure script, since that's still GTK 1.2 and a really don't want to have anything to do with that ;)