configure.ac assumes -std=c11 rather than checking it, which causes make failures on old compilers

[CS-cwhite]

Bug report

Bug description:

Repro

Steps:

1. Run a container having a gcc that does not support the C11 standard (e.g., gcc 4.4.7)
2. tar xvJf Python-3.13.2.tar.xz && cd Python-3.13.2
3. ./configure && make

Expected: configure reports an error like "your compiler doesn't support C11"

Observed: configure succeeds, only for make to fail.

...
configure: creating Makefile
configure:

If you want a release build with all stable optimizations active (PGO, etc),
please run ./configure --enable-optimizations

configure: Your compiler or platform does have a working C11 stdatomic.h. A future version of Python may require stdatomic.h.
sh-4.1# make
gcc -std=gnu99 -pthread -c -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden  -I./Include/internal -I./Include/internal/mimalloc  -I. -I./Include    -DPy_BUILD_CORE -o Programs/python.o ./Programs/python.c
cc1: error: unrecognized command line option "-std=c11"
make: *** [Programs/python.o] Error 1

Suggestion

I think this can be fixed by changing configure.ac here:

-    CFLAGS_NODIST="$CFLAGS_NODIST -std=c11"
+    AX_CHECK_COMPILE_FLAG([-std=c11],[
+        CFLAGS_NODIST="$CFLAGS_NODIST -std=c11"
+    ], [
+        AC_MSG_ERROR([Python requires -std=c11 on gcc])
+    ], [-Werror])

Related issues

#91731

CPython versions tested on:

3.12.9, 3.13.2

Operating systems tested on:

Linux

[picnixz]

PR is welcome!

[vstinner]

cc @encukou