CMake: CMake config files are defining CURL_STATICLIB for static builds

This change allows to use the CMake config files generated by Curl's
CMake scripts for static builds of the library.
The symbol CURL_STATIC lib must be defined to compile downstream,
thus the config package is the perfect place to do so.

Fixes #2817
Closes #2823
Reported-by: adnn on github
Reviewed-by: Sergei Nikulov