cc: Ignore undefined symbols on macOS
Compiling and CC plugin on macOS:
Undefined symbols for architecture arm64:
"_nbdkit_absolute_path", referenced from:
_password_config in test-read-password-plugin-6093ac.o
"_nbdkit_error", referenced from:
_password_config in test-read-password-plugin-6093ac.o
_password_config_complete in test-read-password-plugin-6093ac.o
To work around this we need to tell the linker to ignore undefined
symbols. On macOS libtool uses the same flag already which is why our
regular plugins/filters can be compiled.