mmx.h was distributed separately back in the good old days.
/ Per Hedbor ()
Previous text:
2002-12-06 17:26: Subject: mmx.h
It's part of an "optimisationlibrary", consisting of several headerfiles with inline-versions of functions and in the case of mmx.h, assemblerfunctions for accessing MMX-functions. I found it using google. I think the package is called opts or something similar. The mmx.h file doesn't have any copyrightinformation as far as I can see, but some of the other files included in the package states that they are distributed under GPL2 or later.
/ Marcus Agehall (Trådlös)