Make the macOS bundle compatible osx 10.12

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.

Makefile

@@ -8,6 +8,8 @@ APP_DIR = $(RELEASE_DIR)/osx
 APP_BINARY = $(RELEASE_DIR)/$(TARGET)
 APP_BINARY_DIR  = $(APP_DIR)/$(APP_NAME)/Contents/MacOS
 
+export MACOSX_DEPLOYMENT_TARGET = $(shell defaults read loginwindow SystemVersionStampAsString)
+
 DMG_NAME = Alacritty.dmg
 DMG_DIR = $(RELEASE_DIR)/osx
 
@@ -22,6 +24,9 @@ help: ## Prints help for targets with comments
 
 binary: | $(TARGET) ## Build release binary with cargo
 $(TARGET):
+ifneq ( "${MACOSX_DEPLOYMENT_TARGET}" , "")
+	@echo MACOSX_DEPLOYMENT_TARGET=$${MACOSX_DEPLOYMENT_TARGET}
+endif
 	cargo build --release
 
 app: | $(APP_NAME) ## Clone Alacritty.app template and mount binary