bug#41136: GNUmakefile mode: $Q1: Q, 1 both the same color

[積丹尼 Dan Jacobson]

After the $ signs the E's are properly colored, but not the Q's.
(This is extra bad as the user will not realize he is actually writing
$(Q)1, not $(Q1), if not intentional.)

$ cat Makefile
E=abc
Q=efg
dd:
        $E v.category input=$Q1 output=$Q2 option=add
        $E v.out.ascii format=standard layer=-1 input=$Q2

P.S., Say, perhaps the E and Q before = on lines 1 and 2 should also be
colored too?
emacs-version "26.3"