cmake: fix curl-config --static-libs

The `curl-config --static-libs` command should not output paths like
-l/usr/lib/libssl.so, instead print the absolute path without `-l`.

This also removes the confusing message "Static linking is broken" which
was printed because curl-config --static-libs was disfunctional even
though the static libcurl.a library works properly.

Fixes https://github.com/curl/curl/issues/841