Add RCFLAGS variable in Windows build file, and use it

- Allow user-defined RCFLAGS
- Pass RCFLAGS to RC

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8803)

(cherry picked from commit a583172dac8bce37e268943e570968f193e8b64b)