Move -warnings-as-errors flag from Package.swift to Justfile

This is to work around the fact that Xcode always passes the
incompatible -suppress-warnings flag
when building the library automatically from the Xcode GUI. While it's
possible to suppress this behaviour with xcodebuild on the command line
using an -xcconfig option, there's no evident way to get the Xcode *GUI*
to avoid setting -suppress-warnings when doing a Build or Run action,
short of extensive build configuration which I want to avoid.

Author: mbhutton

Package.swift

@@ -1,7 +1,7 @@
 // swift-tools-version:5.9
 import PackageDescription
 
-let COMPILER_FLAGS = ["-diagnostic-style=llvm", "-warnings-as-errors"]
+let COMPILER_FLAGS = ["-diagnostic-style=llvm"]  // See Justfile for more '-Xswiftc' flags
 
 let package = Package(
     name: "EmacsOpen",

justfile

@@ -11,7 +11,9 @@ format:
 
 build-cli-release:
   @echo "Building CLI..."
-  swift build --configuration release --product emacsopen
+  # Note: define -warning-as-errors here, not in Package.swift, because Xcode will always pass the
+  # incompatible -suppress-warnings flag when building the library automatically from the Xcode GUI.
+  swift build --configuration release --product emacsopen -Xswiftc -warnings-as-errors
 
 build-app-release:
   @echo "Building App using Xcode..."