Support external custom builds of OpenSSL

[agentzh]

Right now the build system can only use its own downloaded OpenSSL, which makes it quite inconvenient while integrating keyless into some of the other projects because other projects might have already built an OpenSSL somewhere itself. Rebuilding OpenSSL over and over again is very time consuming and also very annoying (especially that OpenSSL's build system does not support parallel build via the Gnu make's jobserver feature). To help integrating this server into the automated testing toolchain of other (CF) projects, can we add special make variables to short-cut the openssl building logic in the build system?