The SOURCE_DATE_EPOCH mechanism for reproducible builds require some date
with -u and -d switch to print the date string of another time. In other
words it requires some date that behaves like the GNU date.

Respect this and search a working date, error on no working version.

Signed-off-by: Andreas Bießmann <andreas.de...@googlemail.com>
---
This commit tries to figure out if we have a date variant
available that supports the '-u' and '-d "@0"' switches.
It errors on non-working variants of date.

To respect *BSD host systems search for gdate and date.gnu. Those pre- and
suffixes are widespread used for the GNU variant of a tool also avialable on
*BSD systems.

The result is:

---8<---
abiessmann@punisher % PATH=$ARMv5_PATH:$PATH 
CROSS_COMPILE=arm-v5te-linux-gnueabi- make O=/tmp/picosam ARCH=arm 
include/generated/timestamp_autogenerated.h
make[1]: Entering directory '/tmp/picosam'
  CHK     include/generated/timestamp_autogenerated.h
  UPD     include/generated/timestamp_autogenerated.h
make[1]: Leaving directory '/tmp/picosam'
abiessmann@punisher % PATH=$ARMv5_PATH:$PATH 
CROSS_COMPILE=arm-v5te-linux-gnueabi- make O=/tmp/picosam SOURCE_DATE_EPOCH="0" 
ARCH=arm include/generated/timestamp_autogenerated.h
make[1]: Entering directory '/tmp/picosam'
/home/abiessmann/src/u-boot/Makefile:1304: *** "Your gdate/date.gnu/date does 
not support the '-u' and '-d' switches like GNU date does!".  Stop.
make[1]: Leaving directory '/tmp/picosam'
Makefile:146: recipe for target 'sub-make' failed
make: *** [sub-make] Error 2
--->8---

It applies on top of http://patchwork.ozlabs.org/patch/506856/ (Makefile: Use 
correct timezone for U_BOOT_TZ).

Changes in v2:
* check for '-u' and '-d "@0"' switch rather than for the GNU variant

 Makefile | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index b9b2375..b797e38 100644
--- a/Makefile
+++ b/Makefile
@@ -346,6 +346,10 @@ PERL               = perl
 PYTHON         = python
 DTC            = dtc
 CHECK          = sparse
+DATE          := $(foreach date,gdate date.gnu date, \
+                   $(shell _date=`which $(date)`; \
+                     $${_date} -u -d "@0" >/dev/null 2>&1; \
+                     test $$? -eq 0 && echo $${_date}))
 
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
                  -Wbitwise -Wno-return-void -D__CHECK_ENDIAN__ $(CF)
@@ -1281,9 +1285,9 @@ endef
 define filechk_timestamp.h
        (if test -n "$${SOURCE_DATE_EPOCH}"; then \
                SOURCE_DATE="@$${SOURCE_DATE_EPOCH}"; \
-               LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE "%b 
%d %C%y"'; \
-               LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME 
"%T"'; \
-               LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ 
"%z"'; \
+               LC_ALL=C $(DATE) -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE 
"%b %d %C%y"'; \
+               LC_ALL=C $(DATE) -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME 
"%T"'; \
+               LC_ALL=C $(DATE) -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ 
"%z"'; \
        else \
                LC_ALL=C date +'#define U_BOOT_DATE "%b %d %C%y"'; \
                LC_ALL=C date +'#define U_BOOT_TIME "%T"'; \
@@ -1295,6 +1299,11 @@ $(version_h): include/config/uboot.release FORCE
        $(call filechk,version.h)
 
 $(timestamp_h): $(srctree)/Makefile FORCE
+ifneq ($(strip $(SOURCE_DATE_EPOCH)),)
+ifeq ($(strip $(DATE)),)
+       $(error "Your gdate/date.gnu/date does not support the '-u' and '-d' 
switches like GNU date does!")
+endif
+endif
        $(call filechk,timestamp.h)
 
 # ---------------------------------------------------------------------------
-- 
2.1.4


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