message: make build reproducible
  by allowing build systems to use a constant date instead of the current one
  see for why this is good
  This call is designed to work with both GNU date and BSD date implementations
diff: @@ -22,7 +22,8 @@ CFG_RELEASE=$(CFG_RELEASE_NUM)-dev
  CFG_PACKAGE_VERS=$(CFG_RELEASE_NUM)-dev
  endif
  -CFG_BUILD_DATE = $(shell date +%F)
  +# allow build systems to use a constant date instead of the current one
  +CFG_BUILD_DATE = $(shell SOURCE_DATE_EPOCH="$${SOURCE_DATE_EPOCH:-$$(date +%s)}" ; date -u -d "@$$SOURCE_DATE_EPOCH" +%F 2>/dev/null || date -u -r "$$SOURCE_DATE_EPOCH" +%F 2>/dev/null || date -u +%F)
  ifeq ($(wildcard $(CFG_SRC_DIR)/.git),)
  CFG_VERSION = $(CFG_RELEASE) (built $(CFG_BUILD_DATE))
  