video_chroma: avoid genmf
mmx: merge back into video_chroma (refs #9367)
sse2: merge back into video_chroma (refs #9367)
altivec: merge back into video_chroma (refs #9367)

It is a bit odd that both inline and external flags are mixed together.
This might be an old silent bug.
configure: remove stray references to memcpyaltivec