To make sure that CI builds produced by Travis work on all supported
versions of macOS and building Alacritty works on all supported
versions, the minimum macOS version has been set in the Makefile.
This sets the minimum macOS version to `10.11` as a reasonable
estimation based on other applications. If the exact version chosen
turns out to cause some issues, it can be changed in the future.
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.
