fix, PEBKAC, actually check for the mex header in sxe-matlab m4 macro package

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
chore, refactor onion checks into ext file sxe-onion.m4

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'bld/dist' into next

* bld/dist:
  chore, refactor onion checks into ext file sxe-onion.m4
  fix, PEBKAC, actually check for the mex header in sxe-matlab m4 macro package