Use pugixml from ports, rather than embedded

--- external/CMakeLists.txt.orig	2018-08-11 01:01:24 UTC
+++ external/CMakeLists.txt
@@ -2,4 +2,3 @@
 # package managers are included with the project (in the 'external' folder)
 
 add_subdirectory("nanosvg")
-add_subdirectory("pugixml")