Configure: stop forcing use of DEFINE macros in headers

There are times when one might want to use something like
DEFINE_STACK_OF in a .c file, because it defines a stack for a type
defined in that .c file.  Unfortunately, when configuring with
`--strict-warnings`, clang aggressively warn about unused functions in
such cases, which forces the use of such DEFINE macros to header
files.

We therefore disable this warning from the `--strict-warnings`
definition for clang.

(note for the curious: `-Wunused-function` is enabled via `-Wall`)

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8234)