MT#55283 make the build reproducible

Whilst working on the Reproducible Builds effort [0], we noticed that
rtpengine could not be built reproducibly.

This is because the manpages generated by pandoc contained the current
build date.

An (upstreamable) patch is attached that uses the value from the
SOURCE_DATE_EPOCH environment variable if available.

 [0] https://reproducible-builds.org/

Change-Id: I2eb22dc5f57af69d217fb06c1c126e7e40dbd451

lib/common.Makefile

@@ -38,7 +38,7 @@ $(DAEMONSRCS) $(HASHSRCS):	$(patsubst %,../daemon/%,$(DAEMONSRCS)) $(patsubst %,
 	cat "$<" | sed '/^# /d; s/^##/#/' | \
 		pandoc -s -t man \
 			-M "footer:$(RTPENGINE_VERSION)" \
-			-M "date:$(shell date -I)" \
+			-M "date:$(BUILD_DATE)" \
 			-o "$@"
 
 resample.c codeclib.strhash.c mix.c packet.c:	fix_frame_channel_layout.h

lib/lib.Makefile

@@ -64,3 +64,11 @@ ifneq ($(DBG),yes)
     endif
   endif
 endif
+
+
+DATE_FMT = +%Y-%m-%d
+ifdef SOURCE_DATE_EPOCH
+    BUILD_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
+    BUILD_DATE ?= $(shell date "$(DATE_FMT)")
+endif