document how to override test results

[Bruno Haible]

Hi,

It would be good if the autoconf documentation would state how to override
the results of autoconf macros (without actually changing the autoconf
macros and rebuilding the configure file).

This question has come up on the gnulib list. I think the answer is as
follows, but it's not documented in the autoconf manual: If a building
user wants to override some values of variables determined by autoconf, he
should do that by setting the particular cache variable name while
(re-)running configure, *not* by passing the modified variable values to
'make'. The reasons are:
  1) Some variables are substituted into other files than Makefiles.
  2) Some other AC_SUBSTed variable values may depend on the guessed ones.
  3) When recursing into subdirectories, variables passed on the
     command line are not necessarily passed to subdirectory 'make' runs.
     (GNU make and BSD make do this, but Solaris make does not.)

Jim Meyering also says in [1]:
  > developers seem relatively used to tracing through Makefile*, but
  > those new to autoconf do not always know to look in config.status.

I cannot find this doc under "Manual Configuration" nor "Site Configuration".

Bruno

[1] http://lists.gnu.org/archive/html/bug-gnulib/2009-09/msg00004.html