doc: building documentation updates.

 * Using portable `cmake --build` instead of `make`.
 * Mentioning all crosscompiling Arch PKGBUILDs.
Platform: finally added proper key events to GlutApplication.

The functions were there all the time, I just didn't try hard enough to
find them. The special keys now have different values (the lower 16 bits
are reserved for ASCII), but it is a source-compatible change.