configure: Add -Wno-gnu-variable-sized-type-not-at-end

A Linux headers update to v6.0-rc switches some definitions from GNU
'zero-length-array' extension to the C-standard-defined flexible array
member. e.g.

struct kvm_msrs {
        __u32 nmsrs; /* number of msrs in entries */
        __u32 pad;

-       struct kvm_msr_entry entries[0];
+       struct kvm_msr_entry entries[];
};

Those (unlike the GNU zero-length-array) have some extra restrictions like
'this must be put at the end of a struct', which clang build would complain
about. e.g. the current code

struct {
        struct kvm_msrs info;
        struct kvm_msr_entry entries[1];
} msr_data = { }

generates the warning like:

target/i386/kvm/kvm.c:2868:25: error: field 'info' with variable sized
type 'struct kvm_msrs' not at the end of a struct or class is a GNU
extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
        struct kvm_msrs info;
                        ^
In fact, the variable length 'entries[]' field in 'info' is zero-sized in
GNU defined semantics, which can give predictable offset for 'entries[1]'
in local msr_data. The local defined struct is just there to force a stack
allocation large enough for 1 kvm_msr_entry, a clever trick but requires to
turn off this clang warning.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20220915091035.3897-2-chenyi.qiang@intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -1258,6 +1258,7 @@ add_to nowarn_flags -Wno-string-plus-int
 add_to nowarn_flags -Wno-typedef-redefinition
 add_to nowarn_flags -Wno-tautological-type-limit-compare
 add_to nowarn_flags -Wno-psabi
+add_to nowarn_flags -Wno-gnu-variable-sized-type-not-at-end
 
 gcc_flags="$warn_flags $nowarn_flags"