On macOS 10.12.6, running `make app` creates a bundle that refuses to
start (it complains application is only compatible with more recent
versions of macOS). The binary works great when running it directly.
To resolve this, the `MACOSX_DEPLOYMENT_TARGET` environment
variable is now set automatically from the makefile. This allows
building on macOS 10.12 and earlier, without negatively impacting the
later versions of macOS.