Disallow MSVC from building the parsing code by default because the latest
stable MSVC (VS2015 Update 3) ICEs when trying to compile the `anyOfChars`
combinator.

Note that VS2015 Update 2 and VS2017 can compile the parsing code, with a
little help. To experiment with this, you can override the default
behavior with -DCAPNP_BUILD_TOOLS=ON.