build: Various fixes for openssl build

build: Various fixes for openssl build

This actually worked before because `pkg-config --cflags openssl`
is empty, and the linker was satisfying `-lssl -lcrypto` from our
indirect dependencies.

Also, in fact we *currently* just want `pkg-config libcrypto` i.e.
`-lcrypto`, since we aren't actually using openssl to speak TLS.

This doesn't actually matter on Fedora at least since they're both in the
`openssl-libs` package, but they are separate for a reason.

Closes: #749
Approved by: jlebon