macOS script to build the universal app bundle.

1. Build TuxPaint.app on Intel macOS, rename it TuxPaint-x86_64.app.
2. Build TuxPaint.app on Apple Silicon macOS, rename it TuxPaint-arm64.app.
3. Copy TuxPaint-x86_64.app to the same directory as TuxPaint-arm64.app
   on the Apple Silicon macOS, then run:

     macos/build-universal.sh TuxPaint-x86_64.app TuxPaint-arm64.app

   ... to produce TuxPaint.app that's universal.
4. `make TuxPaint.dmg` to produce TuxPaint.dmg from the universal
   TuxPaint.app.

macos/build-universal.sh

@@ -0,0 +1,142 @@
+#!/usr/bin/env bash
+
+function usage() {
+    cat <<EOF
+Build a macOS universal application bundle from one or more application
+bundles.
+
+Usage: ${SCRIPTNAME} [OPTIONS] TuxPaint-1.app TuxPaint-2.app ...
+
+  TuxPaint-N.app        Bundle(s) to read.  If no bundles are specified, the
+                        following files are read:
+                        $(
+                            for bundle in "${BUNDLES[@]}"; do
+                                printf "\n%26s%s" "" "$bundle"
+                            done
+                        )
+
+  -o OUTBUNDLE          Bundle to create.  [Default=${OUTBUNDLE}]
+
+EOF
+}
+
+
+##############################################################################
+# CONFIG
+
+ARCHS=( arm64 x86_64 )
+BUNDLES=( $(printf "TuxPaint-%s.app\n" "${ARCHS[@]}") )
+OUTBUNDLE=TuxPaint.app
+FORCE=0
+
+
+##############################################################################
+# PROGRAM BEGINS HERE
+
+function main() {
+    local OPTIND OPTARG OPTERR opt
+    local bundle
+    local isok=1
+
+    # Process arguments
+    while getopts "fo:h" opt; do
+        case "$opt" in
+            f)  FORCE=1           ;;
+            o)  OUTBUNDLE=$OPTARG ;;
+            h)  usage && exit 0   ;;
+            *)  isok=0
+        esac
+    done
+    shift $((OPTIND-1))
+
+    # Use the specified bundles
+    if (( $# )); then
+        BUNDLES=( "$@" )
+    fi
+
+    # Sanity check
+    if (( ! isok )); then
+        usage 1>&2
+        exit 1
+    fi
+
+    # Status
+    echo "   * Creating universal app bundle $OUTBUNDLE by combining ${BUNDLES[*]}..."
+
+    # Validate input
+    for bundle in "${BUNDLES[@]}"; do
+        if ! [[ -d "$bundle" ]]; then
+            printf "     -> FAILED: No such input bundle named %s exists\n" "$bundle" 1>&2
+            isok=0
+        fi
+    done
+    (( isok)) || exit 1
+
+    # Validate output
+    if [[ -d "$OUTBUNDLE" ]] && (( FORCE )); then
+        printf "     -> Deleting %s...\n" "$OUTBUNDLE"
+        rm -rf "$OUTBUNDLE" || exitcode=1
+    elif [[ -d "$OUTBUNDLE" ]]; then
+        printf "     -> FAILED: Output bundled named %s already exists, use -f option to recreate.\n" "$OUTBUNDLE" 1>&2
+        isok=0
+    fi
+    (( isok )) || exit 1
+
+    build-universal
+}
+
+
+function build-universal() {
+    local isok=1
+    local i j
+
+    # Create the base package without binaries or libraries
+    echo "     -> Create skeleton app bundle ${OUTBUNDLE}..."     \
+    && cp -a "${BUNDLES[0]}" "$OUTBUNDLE"                         \
+    && rm -f "${OUTBUNDLE}/Contents/MacOS"/*                      \
+    && find "$OUTBUNDLE" -name '*.dylib' -print0 | xargs -0 rm -f \
+    || return 1
+
+    # Create the universal binary for each binary and library
+    for (( i=0; i < ${#BUNDLES[@]}; i++ )); do
+        local filelist=()
+        local file
+
+        # Binaries
+        for file in "${BUNDLES[i]}/Contents/MacOS"/*; do
+            filelist+=( "${file#${BUNDLES[i]}/}" )
+        done
+
+        # Dynamic Libraries
+        while IFS= read -r file; do
+            filelist+=( "${file#${BUNDLES[i]}/}" )
+        done < <(find "${BUNDLES[i]}" -name '*.dylib')
+
+        # Build each as universal
+        for file in "${filelist[@]}"; do
+            local instances=()
+
+            # Skip if already built
+            [[ -e "${OUTBUNDLE}/${file}" ]] && continue
+
+            # Get this file's instances in all bundles
+            for (( j=i; j < ${#BUNDLES[@]}; j++ )); do
+                if [[ -e "${BUNDLES[j]}/${file}" ]]; then
+                    instances+=( "${BUNDLES[j]}/${file}" )
+                fi
+            done
+
+            # Build into $OUTBUNDLE
+            echo "     -> Combine ${file}..."
+            lipo -create -output "${OUTBUNDLE}/${file}" "${instances[@]}" || isok=0
+        done
+    done
+
+    (( isok ))
+}
+
+
+##############################################################################
+# ENTRY POINT
+
+main "$@"