CMakeLists.txt:
Declare the project's version in the cmake project() call so that it may
be exported to the project's cmake integration (used by other projects).

CMakeLists.txt:
Write a ghc_filesystem-config-version.cmake file and install it, so that
other projects may rely on the exact version of ghc_filesystem.
The compatibility mode implies, that any version of ghc_filesystem with
the same major version is feature compatible.

The projects compatibility rule is that minor changes can be api changes, so I changed it to SameMinorVersion.

As the master version is on that number it avoids confusion.