Early exit if find_library() does not find any lib.
As today, the find_library_create_target() cmake macro
blindly continues even if the lib is not found,
adding LIB_PATH-NOTFOUND to the target and making the build
failing anyway later with non obvious reasons.
This change just early exits if the lib is simply not found with a
proper error message.
Fix github issue #3109
Signed-off-by: William Tambellini <wtambellini@sdl.com>
