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

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.