security/p5-Crypt-SSLeay: fix build with clang 15

During an exp-run for llvm 15 (see bug 265425), it turned out that
security/p5-Crypt-SSLeay failed to build with clang 15:

  SSLeay.xs:159:31: warning: call to undeclared function 'SSLv2_client_method'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
              ctx = SSL_CTX_new(SSLv2_client_method());
                                ^
  SSLeay.xs:159:31: error: incompatible integer to pointer conversion passing 'int' to parameter of type 'const SSL_METHOD *' (aka 'const struct ssl_method_st *') [-Wint-conversion]
              ctx = SSL_CTX_new(SSLv2_client_method());
                                ^~~~~~~~~~~~~~~~~~~~~

This is because SSLeay.xs uses the obsolete define OPENSSL_NO_SSL2 to
determine whether OpenSSL still supports SSLv2. Since this define was
removed from OpenSSL 1.1 and later, define it via CFLAGS instead.

PR:		268258
Approved by:	portmgr (tcberner)
MFH:		2022Q4

security/p5-Crypt-SSLeay/Makefile

@@ -20,6 +20,7 @@ USES=		perl5 ssl
 USE_PERL5=	configure
 CONFIGURE_ARGS=	--no-live-tests
 CFLAGS+=	-I${OPENSSLINC}
+CFLAGS+=	-DOPENSSL_NO_SSL2
 
 post-install:
 	${STRIP_CMD} ${STAGEDIR}${PREFIX}/${SITE_ARCH_REL}/auto/Crypt/SSLeay/SSLeay.so