chore(build) add define to build without expat_config.h

src/lxplib.c

@@ -9,7 +9,27 @@
 #include <stdlib.h>
 #include <string.h>
 
-#include "expat_config.h"
+#ifndef LUAEXPAT_SKIP_DTD_SUPPORT
+  // Not defined, so use the standard libexpat header. Some package managers
+  // do not include this header. In that case LUAEXPAT_SKIP_DTD_SUPPORT allows
+  // you to override the option.
+  // 1. ensure you have libExpat 2.4+
+  // 2. try building using LUAEXPAT_SKIP_DTD_SUPPORT=0
+  // 3. try building using LUAEXPAT_SKIP_DTD_SUPPORT=1
+  #include "expat_config.h"
+#else
+  #if LUAEXPAT_SKIP_DTD_SUPPORT == 0
+    // Force the use of DTD support, if your libExpat was compiled without DTD
+    // support, it will cause trouble.
+    #define XML_DTD 1
+  #elif LUAEXPAT_SKIP_DTD_SUPPORT == 1
+    // Skip the use of DTD support.
+    #undef XML_DTD
+  #else
+    #error LUAEXPAT_SKIP_DTD_SUPPORT must be either 0 or 1, or remain unset
+  #endif
+#endif
+
 #include "expat.h"
 #if (XML_MAJOR_VERSION == 2 && XML_MINOR_VERSION < 4) || (XML_MAJOR_VERSION < 2)
 #error Expat 2.4 or newer is required