* doc/autoconf.texi: Some systematic minor improvements, as

* doc/autoconf.texi: Some systematic minor improvements, as
follows.  Use "makefile" when talking about makefiles
generally (which might be named "makefile" or "Makefile" or even
"foo.mk"), "Makefile" when talking about a specific makefile
called "Makefile".  This unclutters the text from weird quotes
(e.g., "`Makefile's" in info mode).  Similarly, use "@var{foo}
values" rather than "@var{foo}s" and similar constructs containing
"}s".  Use "Make rules" rather than "Makefile rules".  Minor
English-language improvements.  Change the prefix "sub-" to "sub"
and "re-" to "re".
Put blank lines around examples more consistently.
Avoid "rather" and "very" as intensifiers.
Avoid "will" as an auxiliary.
(Limitations of Make): Split this node into....
(Portable Make, $< in Ordinary Make Rules, Failure in Make Rules):
(Leading _ in Macro Names, Backslash-Newline-Newline):
(Backslash-Newline Comments, Long Lines in Makefiles):
(Macros and Submakes, The Make Macro SHELL, Comments in Make Rules):
(obj/ and Make, make -k Status, VPATH and Make):
(VPATH and Double-colon, $< in Explicit Rules):
(Automatic Rule Rewriting, OSF/Tru64 Directory Magic):
(Make Target Lookup, Single Suffix Rules, Timestamps and Make):
New nodes, resulting from splitup of Limitations of Make.
All cross-references changed.  Raise the top node from
a section to a chapter, and all subnodes accordingly.
Redo the introductory wording to match the new organization.
(Installation Directory Variables): Use an example that is
closer to what Autoconf actually does.  Mention that VPATH's
value should not contain metacharacters or white space.
(Fortran Compiler): Fix a VPATH bug in an example.
(Leading _ in Macro Names): Mention that this problem is no longer
of practical concern.
(VPATH and Make): Reword the advice to make it clearer
that Autoconf and Automake support VPATH in non-GNU make, but
many packages have bugs in this area.
($< in Explicit Rules): Refer to Build Directories rather
than using a (non-VPATH-safe) example.
(Automatic Rule Rewriting): Mention the sort of disaster that
can ensue with Solaris-style rule rewriting with VPATH.