gawk misparses $expr++ if expr ends in ++

[Paul Eggert]

Here's an example of the problem:

$ gawk 'BEGIN{a=3}{print $$a++++}'
gawk: {print $$a++++}
gawk:               ^ syntax error

But it's not a syntax error, as the expression conforms to the POSIX
spec: it should be treated like '$($a++)++'.

Mawk, Solaris awk (old awk), and Solaris nawk all accept the
expression.  For example:

$ echo '3 4 5 6 7 8 9' | nawk 'BEGIN{a=3}{print $$a++++}'
7

This is with gawk 3.1.5 on Solaris 8 (sparc).