The VS 2017 batch files print message to stdout that mess up the
environment extraction. Try to silence them, and ignore whatever else
it decides to print unconditionally.

Unless the 'visualstudio' variant is disabled in the config by enabling
the `novisualstudio` variant, recipes that use Meson will be built
using Visual Studio.

Cerbero will attempt to find either Visual Studio 2015 or Visual Studio
2017 and set the correct environment to use the MSVC toolchain. This
is done once at startup, and then used by each recipe at build time.

To get this environment in shell form, use the `win32-msvc-shell.cbc`
and `win64-msvc-shell.cbc` config files.

https://bugzilla.gnome.org/show_bug.cgi?id=796641