games/prboom with GL=on fails at configure

[Andriy Gapon]

games/prboom (2.4.7) with GL=on fails at configure, because it can not
find GL headers under /usr/local (i.e. LOCALBASE).

I think this is because of the following:
CONFIGURE_ENV=  CFLAGS+="-I${LOCALBASE}/include" \
                    ^^^^^^
I think this line sets environment variables and there is no += syntax
for that. If I change '+=' to '=' the port builds fine.

-- 
Andriy Gapon