threads: Be less strict when deciding whether to build libssh_threads.
threads: Be less strict when deciding whether to build libssh_threads.

As mentioned in the previous commit, there are cases where
CMAKE_HAVE_THREADS_LIBRARY is not set and pthreads _is_ being used: one can
pass -DTHREADS_HAVE_PTHREAD_ARG=1 to CMake directly so that it just passes
-pthread to the compiler/linker and does not set CMAKE_HAVE_THREADS_LIBRARY.

Since we are only interested in knowing whether any threading library has
been found, we should use CMAKE_THREAD_LIBS_INIT instead (Threads_FOUND
would also work).

Note that, at the moment, there is only a pthreads backend available in
threads/, so if it is not found configuration will fail because CMake will
try to create a library from an empty set of source files.

Reviewed-by: Andreas Schneider <5be00ddc76278cf6077f5047ca3384a88460c671@cryptomilk.org>

sebadoom/libssh,taikoo/libssh,robxu9/libssh,elastichosts/libssh,nviennot/libssh,wangshawn/libssh,DouglasHeriot/libssh,mwgoldsmith/libssh,pouete/libssh,wangshawn/libssh,Distrotech/libssh,robxu9/libssh,DouglasHeriot/libssh,Distrotech/libssh,kedazo/libssh,taikoo/libssh,mwgoldsmith/libssh,wangshawn/libssh,pouete/libssh,elastichosts/libssh,DouglasHeriot/libssh,mwgoldsmith/libssh,wangshawn/libssh,mwgoldsmith/ssh,nviennot/libssh,DouglasHeriot/libssh,nviennot/libssh,pouete/libssh,taikoo/libssh,sebadoom/libssh,robxu9/libssh,elastichosts/libssh,mwgoldsmith/ssh,mwgoldsmith/ssh,Distrotech/libssh,kedazo/libssh,mwgoldsmith/ssh,mwgoldsmith/libssh,Distrotech/libssh,kedazo/libssh,kedazo/libssh,sebadoom/libssh,sebadoom/libssh,pouete/libssh,nviennot/libssh,robxu9/libssh,taikoo/libssh