Allow to override build date

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Also uses UTC to be independent of timezone settings.

Author: bmwiedemann

Makefile

@@ -270,7 +270,7 @@ ${TARGLIB}.so.${MAJOR}: ${TARGLIB}.so.${MAJOR}.${MINOR}
 # file around.  Generate it such that the ident command can find it
 # and strings -a | grep InfiniPath does a reasonable job as well.
 ${TARGLIB}.so.${MAJOR}.${MINOR}: ${${TARGLIB}-objs}
-	date +'char psmi_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > ${lib_build_dir}/_revision.c
+	date -u -d@$${SOURCE_DATE_EPOCH:-$$(date +%s)} +'char psmi_infinipath_revision[] ="$$""Date: %F %R ${rpm_extra_description}InfiniPath $$";' > ${lib_build_dir}/_revision.c
 	$(CC) -c $(BASECFLAGS) $(INCLUDES) _revision.c -o _revision.o
 	$(CC) $(LDFLAGS) -o $@ -Wl,-soname=${TARGLIB}.so.${MAJOR} -shared -Wl,--unique='*fastpath*' \
 		${${TARGLIB}-objs} _revision.o -L$(build_dir)/ipath $(LDLIBS)