* doc/install.texi (Specific, mips-sgi-irix5): Document IRIX 5
	obsoletion, removal.
	Update IDO URL.
	Document GNU as requirement.
	Update configure requirements.
	(Specific, mips-sgi-irix6): Document IRIX 6 < 6.5 obsoletion,
	removal.
	Recomment IRIX 6.5.18+.
	Document IDF/IDL requirement.
	Document GNU as requirement.
	Document GNU ld bootstrap failure.
	Remove freeware.sgi.com reference.

From-SVN: r157901