build: pin K&R-compatible C standard (gnu99) for modern Autoconf/clang

[gburd]

Pin a K&R-compatible C standard so the tree builds on modern toolchains

Berkeley DB is written in K&R (old-style) C: function definitions name their
parameters in a list following the parenthesis, with the prototype supplied
separately via the __P() macro.

C23 (gnu23) removed old-style definitions from the language. Autoconf

= 2.72 AC_PROG_CC probes -std=gnu23 first and adopts it when the compiler
supports it (recent gcc/clang do), so on a modern toolchain every function
definition in the tree becomes a hard compile error
(error: unknown type name 'dbp' ...). The pre-existing
-Wno-deprecated-non-prototype only silenced a warning; under gnu23 these are
errors, so the tree did not build at all with current clang/gcc.

Fix

• Pre-seed the C11/C23 feature-test cache vars to no so AC_PROG_CC adds no
  -std flag, and strip any -std=gnu23/-std=c23 Autoconf still splices into
  $CC.
• Pin -std=gnu99 in CFLAGS (keeps K&R definitions legal and the GNU
  extensions the tree uses).
• Silence the two diagnostics inherent to intentional K&R style
  (-Wno-deprecated-non-prototype, -Wno-knr-promoted-parameter) behind
  compiler-support probes, so unknown-flag compilers are unaffected. All other
  warnings are kept.
• configure regenerated with Autoconf 2.73 (the dev-flake toolchain).

Why standalone

This is a prerequisite for any branch to build on a current toolchain. It is
currently duplicated on perf/aio-stage2 (#24) and perf/cursor-shard;
landing it on master first lets those rebase onto it instead of carrying
conflicting copies of the regenerated configure.

Verified

Clean build on macOS (Apple clang 21) and via the Nix dev shell (gcc 14):
zero errors, zero source warnings.