The autoconf-based build system has room for improvements. The configure.ac script can be simplified in several places by using AS and AC macros or by defining new custom macros.

For example we have a lot of blocks that look like this:

AC_MSG_CHECKING(for chroot)
AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <unistd.h>]], [[void x=chroot]])],
  [AC_DEFINE(HAVE_CHROOT, 1, Define if you have the 'chroot' function.)
   AC_MSG_RESULT(yes)],
  [AC_MSG_RESULT(no)
])

The block has an issue, too. It does not use AC_CACHE to cache the result. The entire block can be replaced by a custom macro that takes care of everything and implements correct caching:

  PY_CHECK_FUNC([chroot], [#include <unistd.h>])

We can also move several library and header checks from setup.py into configure.ac, e.g. check for soundcard.h or gdbm libs and headers.

The first PR adds helper macros, AC_CACHE_CHECK() [1] and AS_VAR_IF() [2]. It also unified internal variables to use format "ac_cv_func_$funcname", "ac_cv_func_lib_$library_$funcname", or "ac_cv_header_$headername_h". "ac_cv" stands for autoconf cached value.

AC_CACHE_CHECK() replaces AC_MSG_CHECKING() and AC_MSG_RESULT(). The syntax is AC_CACHE_CHECK([text], [cache variable], [body]) where body is only excecuted when the cache variable is not set. The body has to set the cache variable to yes or no. Any output and AC_DEFINE must occur outside the body.

AS_VAR_IF() is a nicer way to write if test $variable = value; then; fi.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Caching-Results.html#Caching-Results
[2] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Polymorphic-Variables.html

GH-29441 introduces forward compatibility issues with autoconf 2.71. I took the output of autoupdate and resolved all warnings.

New changeset be3cd5c05d9fb1d1cdb55cb98ca6ef8f866774be by Christian Heimes in branch 'main':
bpo-45723: Detect missing pkg-config (GH-29442)
https://github.com/python/cpython/commit/be3cd5c05d9fb1d1cdb55cb98ca6ef8f866774be

New changeset 57c50c9c7c701a8301c5a89b2b2d050550f62022 by Christian Heimes in branch 'main':
bpo-45723: Add helper macros and more caching to configure.ac (GH-29429)
https://github.com/python/cpython/commit/57c50c9c7c701a8301c5a89b2b2d050550f62022

New changeset 9bd0cf5970997b63d296e30d51e7bb9a15dcabaf by Erlend Egeberg Aasland in branch 'main':
bpo-45723: Add macro for disabling/enabling CC warnings (GH-29466)
https://github.com/python/cpython/commit/9bd0cf5970997b63d296e30d51e7bb9a15dcabaf

New changeset cbab997efb3ba5123dc8d9f706184fa8e634b3ec by Christian Heimes in branch 'main':
bpo-45723: Prepare support for autoconf 2.71 (GH-29441)
https://github.com/python/cpython/commit/cbab997efb3ba5123dc8d9f706184fa8e634b3ec

AC_CHECK_TYPE is obsolete and it's use is discouraged. There are some gotchas we need to check before switching to AC_CHECK_TYPES.

See:
- https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Obsolete-Macros.html
- https://www.gnu.org/software/autoconf/manual/autoconf-2.70/html_node/Generic-Types.html

New changeset 185533639d6eddd42a28f9e3517067bd877e34c5 by Erlend Egeberg Aasland in branch 'main':
bpo-45723: Remove obsolete AC_EXEEXT from configure.ac (GH-29486)
https://github.com/python/cpython/commit/185533639d6eddd42a28f9e3517067bd877e34c5

New changeset 49171aa91ab78c2608a26e5a75cdceab3bad6176 by Erlend Egeberg Aasland in branch 'main':
bpo-45723: Remove dead code for obsolete `--with-dyld` option (GH-29500)
https://github.com/python/cpython/commit/49171aa91ab78c2608a26e5a75cdceab3bad6176

New changeset 76d14fac72479e0f5f5b144d6968ecd9e594db34 by Erlend Egeberg Aasland in branch 'main':
bpo-45723: Improve and simplify more configure.ac checks (GH-29485)
https://github.com/python/cpython/commit/76d14fac72479e0f5f5b144d6968ecd9e594db34

New changeset fc9b62281931da8d20f85d5ed44cfc24f068d3f4 by Christian Heimes in branch 'main':
bpo-45723: Add --with-pkg-config to configure (GH-29517)
https://github.com/python/cpython/commit/fc9b62281931da8d20f85d5ed44cfc24f068d3f4