Add pkg-config configuration file generation

This merge request adds a pkg-config template (CMake/openvds.pc.in) and extends the CMake configuration such that the file is populated with appropriate information like the installation prefix and the OpenVDS version number. The configuration resulting file (openvds.pc is installed into $CMAKE_INSTALL_PREFIX/lib/pkgconfig/. Depending on the installation prefix one might have to extend the environment variable PKG_CONFIG_PATH to include $CMAKE_INSTALL_PREFIX/lib/pkgconfig/ such that pkg-config file can be found by the system.

This make ingestion of OpenVDS easier for projects that do not use CMake as build system and have pkg-config available for obtaining build flags. One example would be projects using Go and its build system. Go has explicit support for pkg-config in the CGO package to obtain the needed flags for compiling and linking of applications to other packages. Would this be something one could add?

I did test this on my machine (Arm64, Mac OSX 12) and it worked nicely.

Main questions:

• Can the configure_file command stay at this location in the CMakeLists.txt?
• Should we document this feature somewhere (README, somewhere else, changelog)?

Closes #150 (closed)