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>

src/scanner.c

@@ -40,13 +40,13 @@
 
 #if HAVE_LIBXML
 #include <libxml/parser.h>
-#endif
-
-#include "wayland-util.h"
 
 /* Embedded wayland.dtd file, see dtddata.S */
 extern char DTD_DATA_begin;
 extern int DTD_DATA_len;
+#endif
+
+#include "wayland-util.h"
 
 enum side {
 	CLIENT,