Add CI for CMake on Linux and MinGW
Adds CI targets for building libxslt with CMake on Linux and MinGW for both shared and static builds.
Linux has separate targets for gcc and clang, while MinGW has targets for 32bit and 64bit builds.
The build results are available as artifacts.
Also switches find_package(LibXml2)
to config mode to properly handle the dependencies specified in the config file generated by the libxml2 CMake build.