Fix -Wexpansion-to-defined warning with clang-3.9

The error message was:

warning: macro expansion producing 'defined' has undefined behavior
[-Wexpansion-to-defined]

http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20160118/147239.html

src/libssl_compat.h

@@ -37,7 +37,13 @@
 
 #include <openssl/opensslv.h>
 
+#if defined(LIBRESSL_VERSION_NUMBER)
+#define LIBRESSL_IN_USE 1
+#else // !defined(LIBRESSL_VERSION_NUMBER)
+#define LIBRESSL_IN_USE 0
+#endif // !defined(LIBRESSL_VERSION_NUMBER)
+
 #define OPENSSL_101_API                                                        \
-  (!defined(LIBRESSL_VERSION_NUMBER) && OPENSSL_VERSION_NUMBER >= 0x1010000fL)
+  (!LIBRESSL_IN_USE && OPENSSL_VERSION_NUMBER >= 0x1010000fL)
 
 #endif // LIBSSL_COMPAT_H