[Debuginfod] Check libcurl via CMake library.

If libcurl was built with CMake, CMake's FindCURL module defers
completely to the included config file. This config file doesn't set any
of the variables that the current check script depends on; it just sets
up an imported CMake target. Accordingly, the smoke test fails, since it
can't find the libcurl (or its static dependencies).

This changes the compile smoke test to refer to the imported library
instead; this should in turn bring in the necessary include and library
directories via the interface properties set up by CMake. This better
mirrors the way libcurl is referred to elsewhere in the CMakeLists.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D121718