CONFLICTS for build, not for install ?

[Marco Molteni]

I am trying to update a port (graphics/enblend) that contains its own
version of a library that is a port too: graphics/vigra.

The source code for enblend does some
#include <vigra/foo>
#include "vigra/bla"

expecting to include the version of vigra contained in enblend itself.
It seems also that it is mixing at random <> and "". The net effect is
that there are 2 cases:
1. if graphics/vigra is not installed, then enblend compiles and
   installs fine.

2. if graphics/vigra is installed, then the compile of enblend 
   breaks.

I tried just setting
CONFLICTS= vigra*
but is seems that CONFLICTS is tested before installing, while I
need it to be tested before building.

Suggestions? I'd like to avoid rewriting all the #include, although
it might be automated with some sed scripting.

thanks
marco
-- 
Very graphic, classical but efficient.