configure: enable runtime cpu detect by default.

with the exception of libpostproc runtime_cpudetect = no is not
supported thus having no as default really is quite odd, it results in
a libpostproc where HAVE_MMX* have very different meaning from the
rest of ffmpeg and it breaks any x86 cpu that doesnt support mmx2
because mmx2 is hardcoded as a result

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>