build: Use -fstrict-flex-arrays=1 if supported

Due to "historical reasons" both gcc and clang treat *all* trailing
arrays members as flexible arrays, this has an evil side effect
of inhibiting bounds checks on such members as __builtin_object_size
cannot say for sure that:

struct {
...
type foo[3];
}

has a trailing foo member of fixed size rather than unspecified.

Ideally we should use -fstrict-flex-arrays as is, but we have to
tolerate kernel uapi headers that use [0] and third party libraries
written in c89 that may use [1] like curl.

meson.build

@@ -451,6 +451,7 @@ possible_cc_flags = possible_common_cc_flags + [
         '-fno-strict-aliasing',
         '-fstack-protector',
         '-fstack-protector-strong',
+        '-fstrict-flex-arrays=1',
         '-fvisibility=hidden',
         '--param=ssp-buffer-size=4',
 ]