Fix adding Opts with | to lvalue Parser

This is the recommended way of adding new Opts in our documentation
for using custom main, but we did not compile the code to see if it
works. We now compile the example as part of the BUILD_EXAMPLES
option.

Fixes #2787

src/catch2/internal/catch_clara.hpp

@@ -673,7 +673,9 @@ namespace Catch {
 
             template <typename T>
             friend Parser operator|( Parser const& p, T&& rhs ) {
-                return Parser( p ) |= CATCH_FORWARD(rhs);
+                Parser temp( p );
+                temp |= rhs;
+                return temp;
             }
 
             template <typename T>