Recover `zconf.h` in `external`

Fixes #106, and is a follow up of what is missed by #97 to fix #48.

* #106 is reproduced on travis by 8429c20  , see [the travis build for it](https://travis-ci.org/utensil/nupic.core/builds/27014332)
* #106 is fixed by recovering `zconf.h` files in `external` from numenta/nupic@1e60936, right before merging #47 which caused #48 and thus #106