Provide a way to selectively enable assertions in release mode

- Throwing an assertion exception for proper stack unwinding, making
  sure destructors are called

- On in Debug mode, in Release, enable via:

      cmake -DCMAKE_BUILD_TYPE=Release -DENABLE_ASSERTIONS=ON

Current problem that I'm seeing is that some code is not catching
exceptions or worse silently swallowing them. Would like to check the
whole pipeline before merging this in.

src/util/assert.cpp

@@ -0,0 +1,29 @@
+#include "util/assert.hpp"
+
+#include <sstream>
+
+namespace
+{
+// We throw to guarantee for stack-unwinding and therefore our destructors being called.
+void assertion_failed_msg_helper(
+    char const *expr, char const *msg, char const *function, char const *file, long line)
+{
+    std::ostringstream fmt;
+    fmt << file << ":" << line << "\nin: " << function << ": " << expr << "\n" << msg;
+    throw osrm::util::assertionError{fmt.str().c_str()};
+}
+}
+
+// Boost.Assert only declares the following two functions and let's us define them here.
+namespace boost
+{
+void assertion_failed(char const *expr, char const *function, char const *file, long line)
+{
+    ::assertion_failed_msg_helper(expr, "", function, file, line);
+}
+void assertion_failed_msg(
+    char const *expr, char const *msg, char const *function, char const *file, long line)
+{
+    ::assertion_failed_msg_helper(expr, msg, function, file, line);
+}
+}