Fix tarball and install, particularly for INSTALL

Getting the INSTALL file generated from the markdown, into the tarball
via CPack and not otherwise installed is a bit tricky. I did this
manually for 5.0, but this automates things. CPack source archives use
a mechanism that acts on whole directories. So we put INSTALL into its
own output directory (as we do for generated HTML), and the
directory-globbing mechanism takes care of it fine. Added some
documentation for why we need to do this.

Added more CPack-time checks that the tarball will be complete.

Prevented installation of .gitattributes.

Change-Id: I77c645faf7b292a085165bdd1598741dae6972a5