This commit fixes a couple issues in the CMake configuration files and uses
native features from CMake:
* libssh-build-tree-settings.cmake is deleted. There was a typo that made
this file unusable, anyway.
* use the macros available in CMakePackageConfigHelpers.cmake to generate
the version file and check that the files exist
* Remove the LIBSSH_THREADS_LIBRARY variable, it used the non-existent
* Fix the in tree build. libssh can be used uninstalled again.
The values were tested after installing the new files and also without running
Signed-off-by: Christophe Giboudeaux <email@example.com>