configure: make the C11 atomics check more robust

C11 atomics allow direct access. This check should prevent the usage
of bogus stdatomic.h available on some systems.

Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -5825,7 +5825,7 @@ check_header asm/types.h
 # it seems there are versions of clang in some distros that try to use the
 # gcc headers, which explodes for stdatomic
 # so we also check that atomics actually work here
-check_builtin stdatomic stdatomic.h "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0)"
+check_builtin stdatomic stdatomic.h "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0); foo += bar"
 
 check_lib advapi32 "windows.h"            RegCloseKey          -ladvapi32
 check_lib ole32    "windows.h"            CoTaskMemFree        -lole32