Install CMake package config file

[tamaskenez]

The PR amends the existing CMake install commands to also install a package configuration file.
In the client program the package can be located and used like this:

find_package(libfswatch ...)
...
target_link_libraries(... libfswatch::libfswatch ...)

[tamaskenez]

opened #306 instead of to better adhere to contributor guidelines (descriptive branch name)