another LINENO problem with the latest Autoconf snapshot

[Paul Eggert]

I ran into this problem with the LINENO stuff in the latest Autoconf CVS.

I ran "sh configure", but interrupted it midway through with
control-C.  As it happens, I interrupted it just as it was generating
configure.lineno.  The file configure.lineno therefore existed and was
newer than configure, but it was incomplete.  The next time I ran
"configure", it used configure.lineno since configure.lineno was
newer; and that failed with a syntax error.