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`).
faed85b8e4