When libressl was linked the libpthread was missing, add it in addition.
Also make the mxsimage tool to use the OpenSSL 1.1 API for the recent
libressl version.
Fixes: 2c192b6916 ("tools/libressl: update to version 2.7.2")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
