Use explicit echo in tests.
Explicitly specify the include install prefix.

Required by CMake 2.8.11.
Fix building test and examples when libssh2 is a DLL.

The tests and examples need the socket library, independently of libssh2 becuase they create sockets themselves.  These were accidentally being inherited from libssh2 before we made the linkage private, but since then they have been missing.  This only showed up on DLL builds because DLLs don't link their dependencies into their dependents.

This change fixes the problem by making the tests and examples explictly link against the sockets libraries.