Why -I flag does not work?

[Victor Porton]

On Saturday, 9 February 2019 at 08:15:36 UTC, Victor Porton wrote:
> $ DFLAGS="-I/usr/local/include/d/librdf -L-L/usr/local/lib" dub 
> build --compiler=dmd --build=unittest
...
> Why does -I flag in DFLAGS does not work? (Ubuntu Linux)

This does not work too:

$ DFLAGS="-I/usr/local/include/d/librdf -L-L/usr/local/lib" dmd 
-unittest source/xmlboiler/options.d

Why?!