Configure: clang: move -Wno-unknown-warning-option to the front

While gcc ignores unknown options of the type '-Wno-xxx', clang by default issues
a warning [-Wunknown-warning-option] (see [3]), which together with '-Werror'
causes the build to fail. This turned out to be a problem on the 1.0.2 stable branch
in the case of the '-Wextended-offsetof' option, which was removed in version 6.0.0,
but needs to be kept here in order to support older clang versions, too (see #9446).

Incidentally, master and 1.1.1 branch already contained the -Wno-unknown-warning-option
option. Due to its special role and its importance, this commit adds an explaining
commit message and moves the option to the front.

[extended tests]

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9447)

(cherry picked from commit 03e5668343078b963cc6544ad7270743de13e514)