Really (I think) fix the sporadic heimdal build failures with high -j

Really (I think) fix the sporadic heimdal build failures with high -j
levels. The root of the problem was that make was attempting to run up
to three concurrent asn1_compile commands to produce the three outputs
that it was declared to produce.  The failure was caused when the
asn1_compiles were started out of sync and a later one was truncating
the files that another thread was trying to copy.  In reality it is
supposed to be run exactly once and all three outputs are produced in
one pass.

Use the same hack as for the parent's Makefile.inc for the compile_et
multi-output rule.