cmake: use imported targets for OpenSSL and zlib

Imported targets are highly preferred over the individual variables
for includes and libs because they will be used in a coherent way
and any spelling mistakes or unavailability won't go unnoticed.

Also it will prevent bugs like conan-io/conan-center-index#16900
or using mismatching header/libs combinations.

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/external_override/CMakeLists.txt

@@ -44,7 +44,7 @@ else ()
         ${libssh_SOURCE_DIR}/src/md_crypto.c
     )
     set(override_libs
-        ${OPENSSL_CRYPTO_LIBRARIES}
+        OpenSSL::Crypto
     )
 endif (WITH_GCRYPT)