Define MODP_CUSTOM constructors as variadic functions

They now match the dh_constructor_t signature.  This is a follow up for
the changes merged with b668bf3f9e and should fix use of MODP_CUSTOM on
Apple's ARM64 platform.

src/libstrongswan/plugins/openssl/openssl_diffie_hellman.c

@@ -193,7 +193,7 @@ METHOD(diffie_hellman_t, destroy, void,
  * Described in header.
  */
 openssl_diffie_hellman_t *openssl_diffie_hellman_create(
-							diffie_hellman_group_t group, chunk_t g, chunk_t p)
+											diffie_hellman_group_t group, ...)
 {
 	private_openssl_diffie_hellman_t *this;
 	const BIGNUM *privkey;
@@ -225,6 +225,9 @@ openssl_diffie_hellman_t *openssl_diffie_hellman_create(
 
 	if (group == MODP_CUSTOM)
 	{
+		chunk_t g, p;
+
+		VA_ARGS_GET(group, g, p);
 		if (!DH_set0_pqg(this->dh, BN_bin2bn(p.ptr, p.len, NULL), NULL,
 						 BN_bin2bn(g.ptr, g.len, NULL)))
 		{