Rewrite of "Quotation Rule Of Thumb"

[Pavel Roskin]

Hello!

This is almost a complete rewrite of the "Quotation Rule Of Thumb" node in
the Autoconf manual.

The existing text illustrated the rule by AC_TRY_LINK, which is an
exception. I don't think it's helpful to the reader.

I started with a very simple example, then I used the exmple from the end
of the old text to demonstrate two ways to use m4 quotes in tests - double
quoting and quadrigraphs.

A table of quadrigraphs is now included. AC_TRY_LINK is mentioned as an
exception near the end.

The new text has been spell-checked and looked at in xdvi. The @group
directives were added to inhibit page breaks in the examples.

ChangeLog:
        * doc/autoconf.texi (Quotation Rule Of Thumb): Document
        quadrigraphs. Better examples. Mostly rewritten.

Regards,
Pavel Roskin