Use config.h to pass DEFAULT_*_FILE instead of compiler switches

This fixes many issues with funny libdirs, e.g on Debian where cmake's libdir defaults to lib/<arch-triplet>, say lib/x86_64-linux-gnu. Unfortunately DEFAULT_PRELOAD_FILE then becomes /usr/local/lib/x86_64-1-gnu/libear.so, which does not exist.

This commit fixes this issue by passing the defines via config.h instead of compiler switches.