Reworked documentation generation and installation.
Details:
- Remove configure flag used to disable the automatic generation of the
documentation when istalling.
- Now 'make html' generates the documentation.
- Now 'make install' does not install the documentation, 'make install-html'
does that. The latter two changes follow newer automake conventions.
- This also fixes the bug that caused the documentation missing from the
tarball.

Merge