Configure: default incpth to usrinc

A previous commit made findhdr use incpth instead of just usrinc;
however, at the moment, incpth is not being filled for non-gcc
compatible compilers, and so broke the build for those.

This commit makes incpth default to usrinc, which means that
if for whatever reason incpth is not filled, findhdr will still
function as it did before.