Configure: accept Windows style compiler options

Configure: accept Windows style compiler options

Currently the Configure command only supports passing UNIX style
options (`-opt`) to the compiler. Passing Windows style options
(`/opt`) yields an error. Fortunately, the compiler accepts both
types of options, nevertheless this commit fixes that discrimination
of Windows users.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9961)

(cherry picked from commit f246f54f18d380791cc60be4aea0fbc7253a9a20)