Add notes about supported AC/AM/LT versions, and add prerequsites for AC_CANONICAL_TARGET and AC_USE_SYSTEM_EXTENSIONS.
Add notes about supported AC/AM/LT versions, and add prerequsites for
AC_CANONICAL_TARGET and AC_USE_SYSTEM_EXTENSIONS.


git-svn-id: 14be032f8f42541b1a281b51ae8ea69814daf20e@1769 4b44e086-7f34-40ce-a3bd-00e031736276

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