[CMake] set CMAKE_INSTALL_PREFIX in CMakeLists

[Micha Renner]

>   SET(CMAKE_INSTALL_PREFIX "/foo/bar" CACHE PATH "Foo install prefix")
> 
> So, without the test to CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT,
> and without the FORCE option.
> 
> Reason: if someone unsets CMAKE_INSTALL_PREFIX on the command-line with
> -U, CMAKE_INSTALL_PREFIX will default to /foo/bar (which is the
> project's default), instead of /usr/local (which is CMake's default).
No, as I mentioned, there was an article of one the CMake-maintainers
who recommended this.

Since www.cmake.org is currently blocked for me, I cannot search. If I
find the article, I let you know.

Greetings

Micha