installation information tweaks

[Jim Wilson]

There was some discussion of this before, but not all parts of the patch
were discussed.

	* install.texi: Mention HP-UX sed problems, needing GNU make for
 	srcdir != builddir.

The HP-UX sed problem has been fixed as previously mentioned.

Looking the GNU make comment, I see that there is a similar comment about
40 lines before that, so I don't think another mention is necessary.

"Make sure you have a version of Make that supports the @code{VPATH}
feature.  (GNU Make supports it, as do Make versions on most BSD
systems.)"

	* config-lang.in: Remove the messages about possible build
	problems -- now either irrelevant to moved to manual.

The hpux and irix6 messages are obsolete.  The GNU make mention is already
in the manual.  The rs6000 problem might still affect some people, but
not very many, so I guess there is no serious problem if we lose it.
I installed this patch.

Jim