GnuTLS cmake check is insufficient

C/C checks in CMakeLists.txt that GnuTLS is available. But it does not check whether GnuTLS version is too old, thus it's possible to have a build configured successfully and fail during the compilation. CMakeLists.txt must either require GnuTLS no older than a specific version (use second argument in find_package) or check with check_c_source_compiles that all necessary files and symbols are present.

Currently this breaks the build in buildbot on valgrind builder.