FortranCInterface: Fix support for VS with Intel toolset

When using a Visual Studio generator with an Intel toolset, such as

-T "Intel C++ Compiler XE 14.0"

the generated FortranCInterface mangling detection project may fail to build due to devenv not working with the /project ALL_BUILD option. This seems to be a bug in devenv or the Intel VS integration. Work around the problem by building with /project FortranCInterface instead. We only need to build this executable and its dependencies within the detection test project anyway.

Fixes: #16519 (closed)