+CC += -ffile-prefix-map=$(CURDIR)=.
+
+dummy != $(M4) /dev/null || printf 'failed'
+ifeq ($(dummy), failed)
+$(error m4 is required to build this package)
+endif
+dummy != printf '%%%%\n' | $(LEX) -o /dev/null || printf 'failed'
+ifeq ($(dummy), failed)
+$(error (f)lex is required to build this package)
+endif
+
+DATE_FMT := +%B %Y
+# To get a reproducible build, we use $(SOURCE_DATE_EPOCH) instead of the
+# current time if this variable is set.
+ifdef SOURCE_DATE_EPOCH
+ DATE := $(shell LC_ALL=C date -u -d '@$(SOURCE_DATE_EPOCH)' \
+ '$(DATE_FMT)' 2>/dev/null || LC_ALL=C date -u '$(DATE_FMT)')
+else
+ DATE := $(shell date '$(DATE_FMT)')
+endif