1. The imported target properties may not contain -D switches in the
INTERFACE_COMPILE_DEFINITIONS property, because CMake will always
prepend them to each item. Therefore we have to remove those switches
and also pass other options (e.g. -fexceptions) in INTERFACE_COMPILE_OPTIONS
property, otherwise CMake creates "mutants" '-D-fexceptions' alike.
2. FindOpenSSL.cmake variable for include directory is
OPENSSL_INCLUDE_DIR, not OPENSSL_INCLUDE_DIRS.