Need option to be able to install pugixml.cpp to include

When consuming pugixml via vcpkg, if the client wishes to use the PUGIXML_HEADER_ONLY option, they are out of luck because pugixml.cpp is not installed to the includes along with pugixml.hpp.

My solution to this in my port overrides was to add an option to the CMakeLists.txt that would also install pugixml.cpp to the includes and then customize the portfile.cmake to add a vcpkg feature, "header-only", but that was meaningless before the CMakeLists.txt patch was available.

This can be done via the vcpkg registry only but I would prefer to patch the CMakeLists.txt rather than have a patch in the registry that has to track along changes to the pugixml CMakeLists.txt.

The design question that I did not address is whether this should suppress generation of a shared object/dll. My CMake-fu is weak so my fumbling attempts at this were lackluster and eventually since it was a port override for only my usage I gave up.

Please consider this for inclusion to the next pugixml version.

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