dependencies across directories

[Rares Vernica]

Hello,

I have a problem with dependencies across directories. Let me describe it:

We have the following directory structure:

/tmp/makefiletest/
  makefile.inc
  A/
    a.h
    a1.h
    makefile
  B/
    b.cc
    makefile

makefile.inc
---
VPATH /tmp/makefiletest

A/makefile
---
a.h: a1.h
  touch $@

B/makefile
---
b.o: A/a.h

/tmp/makefiletest/B> make b.o

The goal of this test if to have b.cc recompiled if a.h or a1.h changes. 
If a.h changes, it works, but if a1.h changes it does not work. I am 
aware that the A/makefile file is not read, and so the dependency 
between a1.h and a.h is not discovered.

We follow this makefile pattern for our codebase, and only recently I 
discovered this "bug" in our pattern. Is there an easy way to fix it 
without braking everything?

Thanks,
Ray