Configure: clang: add -Wno-unknown-warning-option

Configure: clang: add -Wno-unknown-warning-option

Fixes travis build errors due to clang

    error: unknown warning option '-Wno-extended-offsetof'

It seems like '-Wextended-offsetof' was removed from clang in version 6.0.0,
(see [1], [2]). 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 commit adds the '-Wno-unknown-warning-option' option to make clang
behave more relaxed like gcc.

[1] https://reviews.llvm.org/D40267
[2] https://github.com/llvm/llvm-project/commit/52a3ca9e2909
[3] https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option

[extended tests]

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9446)