libssh: fix cross compile with older cmake

libssh: fix cross compile with older cmake

Old version of cmake break cross-compilation when FindThreads is used
because of a try_run call. Add a workaround that avoids try_run.

cmake bug report: https://gitlab.kitware.com/cmake/cmake/issues/16920

Should fix:
http://autobuild.buildroot.net/results/8aa/8aa2bd894416a4508ca7140f8947ff46243f8b78/
http://autobuild.buildroot.net/results/f2c/f2cc965bb134bf4fd9e625eb267b67c949c202fc/

Cc: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>