configure.ac: call AC_SUBST unconditionally with --with-* options

configure.ac: call AC_SUBST unconditionally with --with-* options

Call AC_SUBST unconditionally when specifying --with-* options,
otherwise options like --with-dbusconfdir=DIR or --with-udevdir=DIR have
no effect.

Before this change, configuring with:

  $ mkdir build
  $ ./configure --disable-systemd \
                --prefix=$(pwd)/build \
                --with-dbusconfdir=$(pwd)/build/etc

resulted in the option value to be ignored at "make install" time, with
this error:

  /bin/mkdir: cannot create directory '/dbus-1/system.d': Permission denied

This is what was going on in configure.ac:

  # define the option
  AC_ARG_WITH([dbusconfdir] ... [path_dbusconfdir=${withval}])

  # when --with-dbusconfdir is NOT used
  if (test -z "${path_dbusconfdir}"); then
    ...

    # define the config dir automatically
    path_dbusconfdir="`$PKG_CONFIG --variable=sysconfdir dbus-1`"

    ...

    # set DBUS_CONFDIR
    AC_SUBST(DBUS_CONFDIR, [${path_dbusconfdir}])
  endif

when --with-dbusconfdir=SOMEDIR was used the test above failed, and the
result was that ${path_dbusconfdir} was indeed defined as manually
specified, but DBUS_CONFDIR was not, and the latter was going to be used
in Makefile.am:

  dbusdir = @DBUS_CONFDIR@/dbus-1/system.d

The failure in mkdir can be exposed by the use of the "--prefix" option
and by running "make install" as a normal user; when running "make
install" with the root user /dbus-1/system.d would be happily (and
wrongly) created.

By always setting variables relative to --with-* options (like
DBUS_CONFDIR) the cases when --with-someoption=SOMEDIR are used get
covered.