Meta: Provide a way to only update a file if the output changes

This is only useful for build commands that update their destination in all cases
and thus sometimes confuse cmake into rebuilding everything needlessly.

Meta/write-only-on-difference.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+set -e
+
+if [ "$#" -lt "2" ]; then
+    echo "USAGE: $0 <file> <cmd...>"
+    exit 1
+fi
+
+DST_FILE="$1"
+shift
+
+# Just in case:
+mkdir -p -- "$(dirname -- "${DST_FILE}")"
+
+cleanup()
+{
+  rm -f -- "${DST_FILE}.tmp"
+}
+trap cleanup 0 1 2 3 6
+
+"$@" > "${DST_FILE}.tmp"
+# If we get here, the command was successful, and we can overwrite the destination.
+
+if ! cmp --quiet -- "${DST_FILE}.tmp" "${DST_FILE}"; then
+    # File changed, need to overwrite:
+    mv -f -- "${DST_FILE}.tmp" "${DST_FILE}"
+fi