[build] require linux for DTD validation

This change removes the use of DTD validation (requiring libxml2) for
non-linux builds.

Test: CQ
Change-Id: I37794f8d28fb470cf0715d00bf934bcb42943b4c
diff --git a/BUILD.gn b/BUILD.gn
index 9aa4323..2f190c8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -113,10 +113,12 @@
 
 source_set("scanner_static") {
   visibility = [ ":*" ]
-  deps = [
-    "//third_party/expat",
-    "//third_party/libxml2",
-  ]
+  deps = [ "//third_party/expat" ]
+
+  if (is_linux) {
+    deps += [ "//third_party/libxml2" ]
+    defines = [ "HAVE_LIBXML=1" ]
+  }
 
   sources = [
     "protocol/wayland.dtd.c",
diff --git a/README.fuchsia b/README.fuchsia
index 211ed5b..7eb66c5 100644
--- a/README.fuchsia
+++ b/README.fuchsia
@@ -8,7 +8,7 @@
 
 After updating this library, maintainers should regenerate dependent configuration files:
 1. git clone -n <upstream> && cd wayland && git checkout <commit>
-2. ./autogen.sh --disable-documentation
+2. ./autogen.sh --disable-documentation --disable-dtd-validation
 3. cp ./src/wayland-version.h ${FUCHSIA_DIR}/third_party/wayland/src
 4. cp ./config.h ${FUCHSIA_DIR}/third_party/wayland
 5. ${FUCHSIA_DIR}/third_party/wayland/protocol/wayland.dtd.c.gen.sh ./protocol/wayland.dtd > ${FUCHSIA_DIR}/third_party/wayland/protocol/wayland.dtd.c
diff --git a/config.h b/config.h
index 5f4849c..8b240bb 100644
--- a/config.h
+++ b/config.h
@@ -11,7 +11,7 @@
 #define HAVE_INTTYPES_H 1
 
 /* libxml-2.0 is available */
-#define HAVE_LIBXML 1
+/* #undef HAVE_LIBXML */
 
 /* Define to 1 if you have the <memory.h> header file. */
 #define HAVE_MEMORY_H 1