[Bug other/21629] [doc] gnat version requirements in install.texi

[charlet at gcc dot gnu dot org]

-- 
Bug 21629 depends on bug 21247, which changed state.

Bug 21247 Summary: Cross-building gnat-4.0.0 requires native gnat-4.0.0
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21247

           What    |Old Value                   |New Value
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |WONTFIX

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21629