Makefile.inc1: Make package timestamps reproducible by default

Set package archive timestamps based on most recent source commit
timestamp (approach suggested by bapt).

I'd like to include git metadata in a file included in src tarballs, so
that the build is reproducible (including the hash shown in uname etc.)
outside of a git checkout.  There are still details to be sorted out to
do that, so this is an interim step to improve reproducibility.

Reviewed by:	bapt
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D49165

Makefile.inc1

@@ -597,8 +597,12 @@ PKG_VERSION:=	${_PKG_REVISION}${EXTRA_REVISION:C/[[:space:]]//g}
 .endif	# !defined(PKG_VERSION)
 
 .if !defined(PKG_TIMESTAMP)
+.if !empty(GIT_CMD) && exists(${GIT_CMD}) && exists(${SRCDIR}/.git)
+SOURCE_DATE_EPOCH!=	${GIT_CMD} -C ${SRCDIR} show -s --format=%ct HEAD
+.else
 TIMEEPOCHNOW=		%s
 SOURCE_DATE_EPOCH=	${TIMEEPOCHNOW:gmtime}
+.endif
 .else
 SOURCE_DATE_EPOCH=	${PKG_TIMESTAMP}
 .endif