[openssl-dev] libcrypto.pc needs to list libpthread as a dependency

[Howard Chu]

In OpenSSL 1.1 on Linux (at least) libcrypto now has a dependency on 
libpthread but this is not reflected in the pkgconfig file. As a result, tools 
like CMake fail to detect libcrypto properly when linking against the static 
library. libpthread should be added to the Libs.private line of the pkgconfig 
file.

For example: 
https://github.com/monero-project/monero/issues/2402#issuecomment-327514216

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/