Catch test framework issue: cannot use Catch::Session()

I get this error in a C++ file where I am writing some tests:

error: no member named 'Session' in namespace 'Catch'
        testResult = Catch::Session().run(test_argc, test_argv);
                     ~~~~~~~^

Looking at the catch.hpp single header file, I noticed that the code that should implement the Session() member function is greyed out, probably because of an #ifdef somewhere, which I cannot find.

Is there any macro to set to use the Session class?

Catch versions: 1.5.3 and 1.5.6.

Reference: http://ift.tt/1puB98D