build: Add cube(pp) to install target

build: Add cube(pp) to install target

- Modify cube and cubepp to obtain the texture image data from
  an include file instead of reading a PPM file at runtime.
  (This is the way it works for Android)
  This removes the need to install an image file or otherwise
  make an image file available in a repo build.
- Add cube and cubepp to the install target.

Note: The file handling code is left in place to make it easy
to add a "-texture_file <file>" option so a user can pass in
a texture file.

Fixes #5