ext/openssl/ossl.h: add helper macros for OpenSSL/LibreSSL versions

Add following convenient macros:

 - OSSL_IS_LIBRESSL
 - OSSL_OPENSSL_PREREQ(maj, min, pat)
 - OSSL_LIBRESSL_PREREQ(maj, min, pat)

Author: rhenium

ext/openssl/ossl.h

@@ -42,6 +42,24 @@
 #include <openssl/evp.h>
 #include <openssl/dh.h>
 
+#ifndef LIBRESSL_VERSION_NUMBER
+# define OSSL_IS_LIBRESSL 0
+# ifdef OPENSSL_VERSION_PREREQ
+#  define OSSL_OPENSSL_PREREQ(maj, min, pat) \
+       OPENSSL_VERSION_PREREQ(maj, min) && OPENSSL_VERSION_PATCH >= pat
+#  define OSSL_LIBRESSL_PREREQ(maj, min, pat) 0
+# else
+#  define OSSL_OPENSSL_PREREQ(maj, min, pat) \
+       OPENSSL_VERSION_NUMBER >= (maj << 28) | (min << 20) | (pat << 12)
+#  define OSSL_LIBRESSL_PREREQ(maj, min, pat) 0
+# endif
+#else
+# define OSSL_IS_LIBRESSL 1
+# define OSSL_OPENSSL_PREREQ(maj, min, pat) 0
+# define OSSL_LIBRESSL_PREREQ(maj, min, pat) \
+      LIBRESSL_VERSION_NUMBER >= (maj << 28) | (min << 20) | (pat << 12)
+#endif
+
 /*
  * Common Module
  */