+TIMESTAMP_FORMAT = %Y-%m-%d %H:%M
+
+# use SOURCE_DATE_EPOCH, or else last Git commit date, or else current date
+SOURCE_DATE_EPOCH ?= $(shell test -d ../.git && test `git ls-files -m | wc -l` -eq 0 && git show -s --format=%ct || date +%s)
+
+# get source date string from either GNU / Linux or BSD / Mac OS X style "date"
+SOURCE_DATE_STRING := $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" "+$(TIMESTAMP_FORMAT)" 2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" "+$(TIMESTAMP_FORMAT)" 2>/dev/null || date -u "+$(TIMESTAMP_FORMAT)")
+
+COMMIT_HASH_FILE = confhash.h
+COMMIT_HASH_NONE = "[changed files]"
+
+# get last Git commit hash, if no files were changed
+SOURCE_HASH_STRING ?= $(shell test -d ../.git && test `git ls-files -m | wc -l` -eq 0 && git show -s --format=%h || echo "$(COMMIT_HASH_NONE)")
+
+LIBGAME_DIR = libgame
+LIBGAME = $(LIBGAME_DIR)/libgame.a