Put -lnettle back on the link line when using Nettle

bacc0240 inadvertently stopped
propagating `pkg-config --libs` output into the link line. This didn’t
affect OpenSSL (since configure.ac puts -lcrypto there manually) or
Apple Common Crypto (since it’s not a separate dylib), but it broke
Nettle builds. Fix Nettle builds by ensuring that `pkg-config --libs`
output actually makes it to the linker.