makefile-mode has broken font-locking

[Tim Van Holder]

In current CVS emacs, makefile-mode has serious breakage when it comes
to font-locking:

$ emacs -q
C-x C-f /path/to/some/Makefile
M-x font-lock-mode

At this point, emacs will stop responding and consume 100% CPU.

I haven't been able to debug this properly, so I don't really have any
idea what is causing it, other than that revision 1.90 of make-mode.el
does not exhibit the problem (although it does have fontifications
that differ substantially from the "old" one (and not for the better,
IMHO)).