Fix doc build with doxygen 1.8.0
Fix doc build with doxygen 1.8.0

1.8.0 introduces markdown support but it currently fails to
parse our environment variables documentation properly
(because of special characters). Just disable markdown support,
we lived well without it earlier.

See https://bugzilla.gnome.org/show_bug.cgi?id=670973 for details.



git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@4360 4b44e086-7f34-40ce-a3bd-00e031736276

BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc,BlueBrain/hwloc