Add anything specifying a threads library to ex_libs

Even -pthread gets treated that way.  The reason to do this is so it
ends up in 'Libs.private' in libcrypto.pc.

Fixes #3884

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/5134)