ConfigureChecks: Stop checking for CMAKE_HAVE_THREADS_LIBRARY.

libssh is primarily interested in whether pthreads is present and can be
used. Checking for CMAKE_HAVE_THREADS_LIBRARY is not the same thing, as
there are cases where pthread exists but CMAKE_HAVE_THREADS_LIBRARY is not
set (for example, FreeBSD passes -DTHREADS_HAVE_PTHREAD_ARG=1 to CMake by
default as a way to skip the checks for -lpthread, -lpthreads and others and
tell the build system that -pthread is the one expected to be used).

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