tests: Conditional compilation of tests (including boost-based unit tests)

To compile tests, code needs to be configured with --with-tests option,
otherwise all tests are disabled.

Tests that are based on Boost's unit test framework are enabled only if
the framework is detected during the configure time.

Change-Id: Ia1768e24b088e4f19d2a8799c6d5baeabb2f44c5
diff --git a/tests_boost/test-encode-decode-certificate.cpp b/tests_boost/test-encode-decode-certificate.cpp
index 4743c95..9acc569 100644
--- a/tests_boost/test-encode-decode-certificate.cpp
+++ b/tests_boost/test-encode-decode-certificate.cpp
@@ -9,6 +9,11 @@
 #include <ndn-cpp/security/certificate/certificate.hpp>
 #include <ndn-cpp/security/certificate/public-key.hpp>
 
+#if __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wtautological-compare"
+#endif
+
 #include <cryptopp/cryptlib.h>
 #include <cryptopp/rsa.h>
 #include <cryptopp/files.h>