faed85b8e4
Got caught out by this because simply specifying `DYLIB_CXX_SOURCES` (without specifying `DYLIB_NAME`) resulted in linker errors because the dylib was never built (and linked). We should probably make that a Makefile error (though I haven't audited when exactly not specifying `DYLIB_NAME` is valid; looked like that can happen when we specify `FRAMEWORK`).