Can't build Catch tests

I'm trying to create a unit test file with Catch but following the instructions on the website doesn't seem to work. If I add only the implicit main clang is not able to build the test complaining a main function is missing:

#define CATCH_CONFIG_RUNNER
#include "../lib/catch.hpp"

TEST_CASE("Test Something", "[Test it]") {
    REQUIRE(true);
    REQUIRE(false);
}

[...]

Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [tests/unit/tests] Error 1
make[1]: *** [tests/unit/CMakeFiles/tests.dir/all] Error 2
make: *** [all] Error 2

If I add an explicit main as defined in the Catch website clang has no problems building the binary:

#define CATCH_CONFIG_RUNNER
#include "../lib/catch.hpp"

TEST_CASE("Test Something", "[Test it]") {
    REQUIRE(true);
    REQUIRE(false);
}

int main( int argc, char* const argv[] )
{
    // global setup...

    int result = Catch::Session().run( argc, argv );

    // global clean-up...

    return result;
}