Why -I flag does not work?

[Victor Porton]

For this project: 
https://github.com/vporton/xml-boiler-dlang/tree/85b5587f50617ad1b001c035c72a5780a1e69c24

$ DFLAGS="-I/usr/local/include/d/librdf -L-L/usr/local/lib" dub 
build --compiler=dmd --build=unittest
Performing "unittest" build using dmd for x86_64.
xml-boiler-dlang ~master: building configuration "library"...
source/xmlboiler/options.d(4,8): Error: module `model` is in file 
'rdf/redland/model.d' which cannot be read
import path[0] = source/
import path[1] = ../../.dub/packages/ae-0.0.2331/ae/sys
import path[2] = ../../.dub/packages/ae-0.0.2331/ae/utils
import path[3] = ../../.dub/packages/ae-0.0.2331/ae/net
import path[4] = /snap/dmd/40/bin/../import/druntime
import path[5] = /snap/dmd/40/bin/../import/phobos
dmd failed with exit code 1.
$ ls /usr/local/include/d/librdf/rdf/redland/model.d
/usr/local/include/d/librdf/rdf/redland/model.d
$ dmd --version
DMD64 D Compiler v2.080.1
Copyright (C) 1999-2018 by The D Language Foundation, All Rights 
Reserved written by Walter Bright

Why does -I flag in DFLAGS does not work? (Ubuntu Linux)