generate separate modules for gl, glu, glx; add info target

generate separate modules for gl, glu, glx; add info target

* .gitignore: New file.

* Makefile.am: Add documentation, and only update the generated files
  when asked to.  Add new low-level generated files and generated
  documentation.

* doc/figl.texi: New file.

* doc/low-level-gl.texi:
* doc/low-level-glu.texi:
* doc/low-level-glx.texi: New generated files.

* figl/config.scm.in: Add abs-top-srcdir.

* figl/low-level.inc.scm: Removed.

* figl/low-level/gl.scm:
* figl/low-level/glu.scm:
* figl/low-level/glx.scm: New generated files.

* figl/low-level/support.scm: New runtime support file.

* figl/parse.scm: Update to expose a "gl-definition" data type, and
  expose a parse-to-list interface rather than a fold interface.
  Various hacky documentation tweaks to generate valid stexi for the
  whole binding set.

* generate-low-level-bindings:
* maint/update-low-level-bindings: Move the updater into maint/ dir, and
  only run when asked.  Generate whole modules, and documentation too.