Make build reproducible

It contained a date on when it was build.

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/cversion.c

@@ -69,7 +69,11 @@ const char *SSLeay_version(int t)
 	if (t == SSLEAY_BUILT_ON)
 		{
 #ifdef DATE
+# ifdef OPENSSL_USE_BUILD_DATE
 		return(DATE);
+# else
+		return("built on: reproducible build, date unspecified");
+# endif
 #else
 		return("built on: date not available");
 #endif