build: fix ./configure --disable-dtd-validation

When configured with --disable-dtd-validation:

  CPPAS    src/dtddata.o
src/dtddata.S: Assembler messages:
src/dtddata.S:39: Error: file not found: src/wayland.dtd.embed
Makefile:1520: recipe for target 'src/dtddata.o' failed

This is because the variable name used does not match the implicit
variable name in autoconf.

Fix the variable name, making both --disable-dtd-validation and
--enable-dtd-validation to what they should.

Do not try to build dtddata.S if dtd-validation is disabled. It depends
on wayland.dtd.embed which is created by configure only if
dtd-validation is enabled.

If not building dtddata.S, also make sure the extern definitions in
scanner.c are compiled out.

Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=575212


Reported-by:  <leio@gentoo.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Tested-by: Bryce Harrington <bryce@osg.samsung.com>