Upgrade to GNU Autoconf 2.72

[erlend-aasland]

Feature or enhancement

Autoconf 2.72 was released 2023-12-22. Relevant highlights (some C&P verbatim) from the release announcement:

• Pre-C89 compilers are no longer supported. This should not be a problem for us, as we're using C11.
• AC_USE_SYSTEM_EXTENSIONS now enables C23 Annex F extensions
  by defining __STDC_WANT_IEC_60559_EXT__ (is this a problem?)
• Autoconf now quotes 'like this' instead of `like this'; perhaps we should clean up our quoting in a separate first, so the 2.72 change is easier to review. <= not worth the churn
• Improved compatibility with a wide variety of systems and tools
  including CheriBSD, Darwin (macOS), GNU Guix, OS/2, z/OS, Bash 5.2,
  the BusyBox shell and utilities, Clang/LLVM version 16, the upcoming
  GCC version 14, etc.

The other version requirements (Perl, M4, etc.) should not affect us.

Preliminary actions:

• gh-115765: Use AX_CHECK_DEFINE instead of AC_EGREP_* #116016
• gh-115765: Don't use deprecated AC_CHECK_TYPE macro in configure.ac #115792

Linked PRs

• gh-115765: Upgrade to GNU Autoconf 2.72 #128411
• gh-115765: Document and enforce Autoconf 2.72 requirement #128502

[erlend-aasland]

The AC_CHECK_TYPE issue was addressed previously: #89886 (comment)

[erlend-aasland]

cc. @corona10

[erlend-aasland]

Seems our usage of AC_EGREP_*() is pretty safe, so it is not imperative to replace them with more modern equivalents.

[erlend-aasland]

Seems our usage of AC_EGREP_*() is pretty safe, so it is not imperative to replace them with more modern equivalents.

Well, the AC_EGREP_HEADER usage is not very clean. We should fix that.