Run emacs -Q /tmp/configure.ac, paste in the following lines:

AC_DEFUN([FOO_DEP],
dnl AC_ARG_WITH([lang$1],
dnl   [
dnl     AS_HELP_STRING([--with-lang$1],
dnl     [Source directory for $2, otherwise uses installed files])
dnl   ],
dnl   [
dnl     SRC$1=$withval
dnl     echo "Using $2 from $SRC$1"
dnl   ],
dnl   [ 
dnl     PKG_CHECK_MODULES([$2], [$2])
dnl     SRC1=`pkg-config --variable=srcdir $2`
dnl   ])
)

then mark the dnl-ed lines, and do M-x uncomment-region. 

Expected result:

AC_DEFUN([FOO_DEP],
AC_ARG_WITH([lang$1],
  [
    AS_HELP_STRING([--with-lang$1],
    [Source directory for $2, otherwise uses installed files])
  ],
  [
    SRC$1=$withval
    echo "Using $2 from $SRC$1"
  ],
  [ 
    PKG_CHECK_MODULES([$2], [$2])
    SRC1=`pkg-config --variable=srcdir $2`
  ])
)

Actual result:

AC_DEFUN([FOO_DEP],
dnl AC_ARG_WITH([lang$1],
  [
dnl     AS_HELP_STRING([--with-lang$1],
    [Source directory for $2, otherwise uses installed files])
dnl   ],
  [
dnl     SRC$1=$withval
    echo "Using $2 from $SRC$1"
dnl   ],
  [ 
dnl     PKG_CHECK_MODULES([$2], [$2])
    SRC1=`pkg-config --variable=srcdir $2`
dnl   ])
)



In GNU Emacs 24.3.1 (x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2013-07-27 on roseapple, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11403000
System Description:	Ubuntu 13.10

Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=lucid' '--with-toolkit-scroll-bars' '--without-gconf'
 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
 'LDFLAGS=-Wl,-Bsymbolic-functions -Wl,-z,relro'
 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LANG: nn_NO.UTF-8
  locale-coding-system: utf-8
  default enable-multibyte-characters: t

Major mode: Autoconf

Minor modes in effect:
  diff-auto-refine-mode: t
  shell-dirtrack-mode: t
  recentf-mode: t
  display-time-mode: t
  show-paren-mode: t
  cua-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t


-- 
Kevin Brubeck Unhammer

GPG: 0x766AC60C