If OPENSSL_BUILD_SHLIBCRYPTO (for files that end up as libcrypto

objects) or OPENSSL_BUILD_SHLIBSSL (for files that end up as libssl
objects) is defined, redefine OPENSSL_EXTERN to be OPENSSL_EXPORT.
This is actually only important on Win32, and can safely be ignored in
all other cases, at least for now.