XML: Add tooling for the pugixml parser

thirdparty/CMakeLists.txt

 add_subdirectory(KWSys)
 add_subdirectory(NLohmannJson)
+add_subdirectory(pugixml)
 
 if(BUILD_TESTING)
   add_subdirectory(GTest)

thirdparty/pugixml/CMakeLists.txt

+# This is an extremely simple and tiny library.  Rather than build it
+# seperately we'll just create an interface library containing the sources and
+# headers
+add_library(pugixml INTERFACE)
+target_sources(pugixml INTERFACE
+  ${CMAKE_CURRENT_SOURCE_DIR}/pugixml/src/pugiconfig.hpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/pugixml/src/pugixml.hpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/pugixml/src/pugixml.cpp
+)
+target_include_directories(pugixml INTERFACE
+  ${CMAKE_CURRENT_SOURCE_DIR}/pugixml/src
+)

thirdparty/pugixml/Readme.txt

+This is a copy of the upstream pugixml library located at
+https://github.com/zeux/pugixml. Do not make changes directly to this repo
+but instead to the upstream repository.  Update this copy of pugixml by running
+the update.sh script.

thirdparty/pugixml/update.sh

+#!/usr/bin/env bash
+
+set -e
+set -x
+shopt -s dotglob
+
+readonly name="pugixml"
+readonly ownership="pugixml Upstream <robot@adios2>"
+readonly subtree="thirdparty/pugixml/pugixml"
+readonly repo="https://github.com/zeux/pugixml.git"
+readonly tag="v1.8.1"
+readonly shortlog="true"
+readonly paths="
+src/pugiconfig.hpp
+src/pugixml.hpp
+src/pugixml.cpp
+"
+
+extract_source () {
+    git_archive
+}
+
+. "${BASH_SOURCE%/*}/../update-common.sh"