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.
diff --git a/tinyxml2.cpp b/tinyxml2.cpp
index 9f3475e..0c903fe 100755
--- a/tinyxml2.cpp
+++ b/tinyxml2.cpp
@@ -2037,7 +2037,7 @@
 		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 @@
     _attributePool.Trace( "attribute" );

 #endif

     

-#ifdef DEBUG

+#ifdef TINYXML2_DEBUG

     if ( !hadError ) {

         TIXMLASSERT( _elementPool.CurrentAllocs()   == _elementPool.Untracked() );

         TIXMLASSERT( _attributePool.CurrentAllocs() == _attributePool.Untracked() );