Make intermediate header file

Instead of making (and depending on) multiple header files in external
repositories, this commit makes single intermediate file and copies it
to external directories on install.

This will...

*   Reduce CPU time (only slightly, though).
*   Enable making header file *without* cloning and destroying
    external repositories (`make encoding.out.h`).
*   Ensure that latest encoding.h is generated and copied on install,
    even after fresh checkout on an external source tree.