Use curl::curl target when available

[ihnorton]

Build system integration could be simplified a bit if this library would conditionally link against the curl::curl target provided by curl's own CMake config files.

Currently the linkage here is unconditionally against CURL_LIBRARIES and CURL_INCLUDE_DIRS, which are only provided by the CMake built-in FindCurl.cmake. (using curl's CMake can be disabled by setting CURL_NO_CURL_CMAKE, but this is only available in CMake 3.17+).

(I'm happy to provide a patch to this effect, but unclear if patches are being accepted/merged here)