Fixed some compile issues with show_library_versions()

Fixed some compile issues with show_library_versions()

* Refactored show_library_versions to work around the fact
  that some compilers (such as MSVC 2008) can't handle #ifdefs
  inside of macro references.

* Declare show_library_versions() in options.h because it's
  referenced by other files such as openvpn.c.

* Declare get_ssl_library_version() as returning const char *,
  to avoid loss of const qualifier in ssl_openssl.c.

Signed-off-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1399589436-8730-7-git-send-email-james@openvpn.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8711
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a91a2d6ad7b139ec78d61c8616b8447847e9ecc6)