use faketime, if available, to set the build time based on the git commit

If the `faketime` utility is available, use it to fix the time of the build
based on the time of the current git commit.  This makes the build process
reproducible since anyone who builds it from git will end up with a build
that has the same exact timestamps.

You can get faketime from various package systems:
 apt-get install faketime
 brew install faketime
 port install libfaketime

Author: eighthave

Makefile

@@ -13,6 +13,22 @@ RELEASE_DIR := sqlipher-for-android-${LATEST_TAG}
 CHANGE_LOG_HEADER := "Changes included in the ${LATEST_TAG} release of SQLCipher for Android:"
 README := ${RELEASE_DIR}/README
 
+# Use faketime to freeze time to make for reproducible builds.
+# faketime needs to have a very specific timestamp format in order to freeze
+# time.  The time needs to be frozen so that the timestamps don't depend on
+# the speed of the machine that the build process is running on.  See `man
+# faketime` for more info on the "advanced timestamp format".  Also, force
+# time to UTC so its always the same on all machines.
+ifeq ($(shell if which faketime > /dev/null; then echo faketime; fi),faketime)
+  export TZ=UTC
+  TIMESTAMP := $(shell faketime "`git log -n1 --format=format:%ai`" \
+                   date --utc '+%Y-%m-%d %H:%M:%S')
+# frozen time
+  FAKETIME := faketime -f "$(TIMESTAMP)"
+# time moving at 5% of normal speed
+  FAKETIME_5 := faketime -f "@$(TIMESTAMP) x0.05"
+endif
+
 init:
 	git submodule update --init
 	android update project -p .
@@ -23,16 +39,16 @@ all: build-external build-jni build-java copy-libs
 
 build-external:
 	cd ${EXTERNAL_DIR} && \
-	make -f Android.mk build-local-hack && \
-	ndk-build NDK_LIBS_OUT=$(EXTERNAL_DIR)/libs && \
-	make -f Android.mk copy-libs-hack
+	$(FAKETIME) make -f Android.mk build-local-hack && \
+	$(FAKETIME) ndk-build NDK_LIBS_OUT=$(EXTERNAL_DIR)/libs && \
+	$(FAKETIME) make -f Android.mk copy-libs-hack
 
 build-jni:
 	cd ${JNI_DIR} && \
-	ndk-build NDK_LIBS_OUT=$(JNI_DIR)/libs
+	$(FAKETIME) ndk-build NDK_LIBS_OUT=$(JNI_DIR)/libs
 
 build-java:
-	ant release
+	$(FAKETIME_5) ant release
 
 release:
 	-rm -rf ${RELEASE_DIR}