configure: make the check for stdatomic.h stricter

Old ICC verions don't advertise having a full C11 implementation but
may nonetheless include a feature-incomplete stdatomic.h header.

Fixes ticket #6049

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

configure

@@ -5623,7 +5623,7 @@ 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_h stdatomic.h "atomic_int foo; atomic_store(&foo, 0)"
+check_builtin stdatomic_h stdatomic.h "atomic_int foo, bar = ATOMIC_VAR_INIT(-1); atomic_store(&foo, 0)"
 
 check_lib2 "windows.h shellapi.h" CommandLineToArgvW -lshell32
 check_lib2 "windows.h wincrypt.h" CryptGenRandom -ladvapi32