Fold README.IRIX5.3 into the IRIX README; we don't have multiple
READMEs per OS, and it's clutter, especially given the information is
from 2006.
Add last-updated dates (2005/2006) as a clue.
PKGSRC_COMPILER=mipspro
if this compiler is used. Otherwise, wrong CFLAGS might be passed.
XXX: we probably want to add a check for the proper compiler into the
bootstrap process and add this line automatically, if necessary.
