configure.ac: first arg to AC_DEFUN needs quotes

automake manual[1] says that "Starting with Automake 1.8,
aclocal will warn about all underquoted calls to AC_DEFUN."
1.13.4 on openSUSE-13.1 really does, while 1.14.1 on archlinux
does not.  go figure.

[1] http://www.gnu.org/software/automake/manual/automake.html#Extending-aclocal