cmake: install a fitting config header

[nolange]

use a header with the correct defines, so
the defines fit to the compiled library.

I have to use this when creating a sysroot / sdk for later builds. the options are currently just discarded.

Ideally the build would use the header too instead of defines, but there is no clean way,
as #include "pugiconfig.hpp" will pretty consistentely prefer the file next to it.
I could change this to always create from CMake, but I guess that breaks all the other build systems.

[zeux]

This can't be unconditional. It needs to be gated under PUGIXML_INSTALL_CONFIG option; I'm not sure if it's better to change PUGIXML_HEADERS or just adjust the installation code. Also at this point I wonder if the sysroot build even needs to use CMake; it looks like your use case requires a very custom configuration and a custom config file.