This allows cargo metadata to operate on it without adding the
source files to the build. (A choice which will save a few rebuilds
of the manual)
(cherry picked from commit 1779295f3e13cc15f8422d52a3753bb927ac8fa7)
See README. Most of this is in accordance with the bindgen introduction.
The c_headers function was taken from Zach Mitchell's work on bindings.
Co-authored-by: Zach Mitchell <zmitchell@fastmail.com>
(cherry picked from commit 83d8ae6b26d341bbd452a77d7361f58c06a2d322)
