cmake-native: Fix host tool contamination (Bug: 14951)

Trying to build cmake-native on a host system where curl was built with cmake
(resulting in CURLConfig.cmake and friends, which do not use the same naming
schemes expected by cmake-native's build process, being installed to a system
wide cmake directory like /usr/lib64/cmake/CURL) results in undefined
references to all libcurl symbols.

The problem is that cmake-native sees and uses the system wide
/usr/lib64/cmake/CURL/CURLConfig.cmake, which defines CURL::libcurl and
CURL::curl as opposed to setting ${CURL_LIBRARIES} as expected by
cmake-native.

find_package(CURL) (cmake-native's CMakeLists.txt, line 478) succeeds, but
incorrectly uses the system wide CURLConfig.cmake, resulting
CMAKE_CURL_LIBRARIES to be set to an empty string (cmake-native's
CMakeLists.txt, line 484), causing the cmake-native build to miss -lcurl.

The simplest fix is to let cmake know the right value for
CURL_LIBRARIES. Making it -lcurl should always work with libcurl-native
in recipe-sysroot-native.

Signed-off-by: Bernhard Rosenkränzer <bero@baylibre.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>