bug#10033: 24.0.91; Uncommenting fails in Autoconf mode

[Chong Yidong]

With latest trunk:

emacs -Q configure.ac
M-;
asdf
C-SPC M-<
M-x uncomment-region RET

The uncomment-region does nothing; the line containing "dnl asdf"
remains commented out.

This is a regression; in Emacs 23.3, uncomment-region works as expected.
However, using the version of autoconf.el from Emacs 23 in Emacs 24
still shows the problem.

In GNU Emacs 24.0.91.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.2.0)
 of 2011-11-13 on furball
Windowing system distributor `The X.Org Foundation', version 11.0.11004000
configured using `configure  '--with-x-toolkit=gtk3' 'CFLAGS=-g''