Pick up date from SOURCE_DATE_EPOCH, for reproducible builds

The goal of reproducible builds is that a rebuild of the same source
code with the same compiler, libraries, etc. should result in the same
binaries. SOURCE_DATE_EPOCH provides a standard way for build systems
to fill in the date of the latest source change, typically from a git
commit or from metadata like the debian/changelog in Debian packages.

This does not change anything if SOURCE_DATE_EPOCH is not defined;
the intention is that a larger build system like a Debian package
will define it.

Please see https://reproducible-builds.org/ for more information about
reproducible builds.

Makefile

@@ -1098,6 +1098,11 @@ else
   STRIP_FLAG = -s
 endif
 
+# https://reproducible-builds.org/specs/source-date-epoch/
+ifdef SOURCE_DATE_EPOCH
+  BASE_CFLAGS += -DPRODUCT_DATE=\\\"$(shell date --date="@$$SOURCE_DATE_EPOCH" "+%b %_d %Y" | sed -e 's/ /\\\ /'g)\\\"
+endif
+
 BASE_CFLAGS += -DPRODUCT_VERSION=\\\"$(VERSION)\\\"
 BASE_CFLAGS += -Wformat=2 -Wno-format-zero-length -Wformat-security -Wno-format-nonliteral
 BASE_CFLAGS += -Wstrict-aliasing=2 -Wmissing-format-attribute