[screen-devel] Fix out of tree build

[Kieran Bingham]

Whilst trying to build screen, I attempted a build the master branch
in a sub-directory (src/build) to allow clean source tree.

This failed, and it would appear to result from commit
144836c3bb020405bf029d0d5a85c84aa74662d9 when the -I flags were
removed.

As yet, I can not see why these flags were removed, and perhaps 
there is a better way to fix out of tree build, or indeed
perhaps I am simply doing something else wrong (Does this work for
anyone else?)

Otherwise, the following patch fixes things. If this is appropriate,
please apply

Tested for both in-tree and out-of-tree build
--
Regards

Kieran