cmake 3.4 is required because of string(APPEND ...)

When trying to compile libssh-0.10.5

CMakeLists.txt and INSTALL require cmake 3.3.0. But cmake 3.3.0 fails:

CMake Error at ConfigureChecks.cmake:332 (string): string does not recognize sub-command APPEND

https://cmake.org/cmake/help/latest/command/string.html#append

APPEND requires cmake 3.4.

You could either require version 3.4 or rewrite the append into something like:

set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -Wattributes")