rename DEBUG define to TINYXML2_DEBUG

The original name is to general. This prevents possible name collisions with other defines, enums, etc. from projects where tinyxml2 is used.

tinyxml2.cpp

@@ -2037,7 +2037,7 @@ void XMLDocument::Clear()
 		DeleteNode(_unlinked[0]);	// Will remove from _unlinked as part of delete.
 	}
 
-#ifdef DEBUG
+#ifdef TINYXML2_DEBUG
     const bool hadError = Error();
 #endif
     ClearError();
@@ -2052,7 +2052,7 @@ void XMLDocument::Clear()
     _attributePool.Trace( "attribute" );
 #endif
     
-#ifdef DEBUG
+#ifdef TINYXML2_DEBUG
     if ( !hadError ) {
         TIXMLASSERT( _elementPool.CurrentAllocs()   == _elementPool.Untracked() );
         TIXMLASSERT( _attributePool.CurrentAllocs() == _attributePool.Untracked() );