[PATCH 0/2] Modernize header checks

[Zack Weinberg]

This patch series accomplishes two things.

First, it switches AC_CHECK_HEADER[S] to do only a compile test by
default.  This completes the transition begun in Autoconf 2.56.  It
has been my experience that basically nobody bothered to disable the
"mongrel" mode, and that basically nobody actually needed the
preprocessor check.  Therefore, I do not anticipate problems from this
change.  Mongrel mode is completely removed.  Preprocessor-only checks
are still available by specifying [-] as the fourth argument to
AC_CHECK_HEADER, but this will issue a deprecation warning and the
manual tells people to switch to AC_PREPROC_IFELSE.

Second, it cleans up AC_INCLUDES_DEFAULT and all the other canned
tests so that they don't waste time checking for ISO C90 headers,
which are now ubiquitous (stddef.h, stdlib.h, string.h, wchar.h,
wctype.h, locale.h, time.h) and don't use pre-standard headers that
were replaced by C90 headers at all (memory.h and strings.h).  Certain
very old macros (AC_HEADER_STDC, AC_HEADER_TIME, AC_USG, AC_MEMORY_H)
are more aggressive about autoupdating themselves out of existence,
autoscan won't tell you you need to probe for C90 headers anymore, and
finally, the manual is revised so that it does not recommend checking
for obsolete or C90 headers.  (I may not have found every last
instance of such obsolete advice in the manual.)

I *should* have a past-and-future-changes copyright assignment on
file, but I'm not sure whether it explicitly covered Autoconf.

zw

 NEWS                      |   24 +++
 doc/autoconf.texi         |  396 +++++++++++++++++----------------------------
 lib/autoconf/functions.m4 |   35 +----
 lib/autoconf/headers.m4   |  281 ++++++++++-----------------------
 tests/c.at                |   28 +++-
 tests/local.at            |    5 +-
 tests/semantics.at        |   17 +-
 tests/tools.at            |    6 +-
 8 files changed, 297 insertions(+), 495 deletions(-)