build: add autotools bootstrap skeleton with C23 flags

configure.ac

@@ -0,0 +1,27 @@
+# configure.ac - stupidtools bootstrap (autotools, C23).
+#
+# This is the *bootstrap* build only: stupidtools will later generate its
+# own ./configure (self-hosting, todo 23). Only Makefile.am inputs are
+# tracked in git; configure/Makefile* are generated and gitignored.
+
+AC_PREREQ([2.71])
+AC_INIT([stupidtools], [1.0.0], [[email protected]])
+AM_INIT_AUTOMAKE([foreign subdir-objects])
+AC_CONFIG_SRCDIR([src/main.c])
+AC_PROG_CC
+
+# Require a compiler that actually accepts C23 (ISO/IEC 9899:2024).
+# -std=c23: GCC >= 14, Clang >= 18. Never -std=c2y (next draft, not ratified).
+save_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -std=c23 -Werror"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+  [[#if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 202311L
+#error "C23 (__STDC_VERSION__ >= 202311L) required"
+#endif]],
+  [[_Static_assert(1, "ok");]])],
+  [],
+  [AC_MSG_ERROR([a C compiler with C23 support (GCC >= 14 or Clang >= 18) is required])])
+CFLAGS="$save_CFLAGS"
+
+AC_CONFIG_FILES([Makefile src/Makefile])
+AC_OUTPUT