Prefer AM_CPPFLAGS to @CPPFLAGS@

[Stepan Kasal]

Hello,
  I suggest a change to the Autoconf manual, see the patch below.

For people which use the standard set of Autotools, this can be
pasted to the project, while the previous version was not optimal
(breaking override of CPPFLAGS for "make").

For the rare species of Autoconf without Automake users, this does
not hurt, as it does hide any info.  Those uses know well how to
``extend CPPFLAGS.''

OK to commit?

Stepan

autoconf.texi
Description: TeXInfo document