[XEN PATCH 2/5] docs: set DATE to SOURCE_DATE_EPOCH if available

[Maximilian Engelhardt <maxi@xxxxxxxxxxxxx>]

Use the solution described in [1] to replace the call to the 'date'
command with a version that uses SOURCE_DATE_EPOCH if available. This
is needed for reproducible builds.

The -d "@..." syntax was introduced in GNU date about 2005 (but only
added to the docuemntation in 2011), so I assume a version supporting
this syntax is available, if SOURCE_DATE_EPOCH is defined. If
SOURCE_DATE_EPOCH is not defined, nothing changes with respect to the
current behavior.

[1] https://reproducible-builds.org/docs/source-date-epoch/

Signed-off-by: Maximilian Engelhardt <maxi@xxxxxxxxxxxxx>
---
 docs/Makefile | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/docs/Makefile b/docs/Makefile
index b30cc619f8..beba02a94f 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -3,7 +3,13 @@ include $(XEN_ROOT)/Config.mk
 -include $(XEN_ROOT)/config/Docs.mk
 
 VERSION                := $(shell $(MAKE) -C $(XEN_ROOT)/xen 
--no-print-directory xenversion)
-DATE           := $(shell date +%Y-%m-%d)
+
+DATE_FMT       := +%Y-%m-%d
+ifdef SOURCE_DATE_EPOCH
+DATE           := $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" "$(DATE_FMT)" 
2>/dev/null || date -u -r "$(SOURCE_DATE_EPOCH)" "$(DATE_FMT)" 2>/dev/null || 
date -u "$(DATE_FMT)")
+else
+DATE           := $(shell date "$(DATE_FMT)")
+endif
 
 DOC_ARCHES      := arm x86_32 x86_64
 MAN_SECTIONS    := 1 5 7 8
-- 
2.39.5