Fix tarball and install, particularly for INSTALL

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