Use CURL::libcurl imported target when available

When built with CMake, CURL installation includes configuration package
that could be used instead of CMake-bundled find module. The former
doesn't define CURL_INCLUDE_DIRS and CURL_LIBRARIES variables, but only
imported targets.

CMakeLists.txt

@@ -135,6 +135,10 @@ find_package(Threads)
 find_package(PkgConfig QUIET)
 
 find_package(CURL ${CURL_MINIMUM} REQUIRED)
+if(TARGET CURL::libcurl)
+    set(CURL_LIBRARIES CURL::libcurl)
+    set(CURL_INCLUDE_DIRS)
+endif()
 
 find_package(ICONV)