Flex 2.5.31 m4 bug

[Bruce Lilly]

I tried flex 2.5.31.  Much to my surprise, on running flex
on a 1,132-line input file that has worked fine with lex and
with flex versions from at least 2.5.2 on, I get:

m4: ERROR: EOF in string
m4: ERROR: EOF in string

No line numbers or any meaningful information, and as I had
run flex, not m4, the error messages were quite surprising!

I have been able to reproduce the bug with the following
simple input file:

========================================================
%%
    /* message in -> [unstuff] -> parse -> [[stuff] -> message out] */
%%
========================================================

Flex seems to be unreasonably objecting to the perfectly
valid C comment (which is one of the 1,132 lines from the
aforementioned file).  And there's nothing there that could
reasonably be called a "string". What an "EOF in string"
is supposed to be is a great mystery, since the phrase
seems to be an oxymoron.  How one could possibly have *two*
such things in a single file containing a single comment
is an even bigger mystery.