build.xml: Add SOURCE_DATE_EPOCH support

Set the "timestamp" attribute to the current time. When using
Ant >= 1.10.8, it can be overridden with the SOURCE_DATE_EPOCH
environment variable. With Ant >= 1.10.2, the ant.tstamp.now
property does the same thing.

Use <jar modificationtime="${timestamp}"> to set file timestamps
inside JARs.

Reproducible builds should now be possible by setting SOURCE_DATE_EPOCH
to a known value. Timestamps in JARs are in local timezone, thus
setting also TZ=UTC0 can be useful. For example, the committer date
of the latest commit provides a reasonable timestamp:

    $ SOURCE_DATE_EPOCH=$(git log -n1 --pretty=%ct) TZ=UTC0 ant

README.md

@@ -32,6 +32,11 @@ Building with Apache Ant
 
   Notes about old build environments:
 
+  * If you are using Ant older than 1.10.2:
+
+    Edit `build.xml` and remove the `modificationtime="${timestamp}"`
+    attributes from the `<jar>` elements.
+
   * If you are using Ant older than 1.9.8:
 
     Edit `build.xml` and remove the release attributes from the

build.xml

@@ -8,6 +8,10 @@
 
     <property file="build.properties"/>
 
+    <tstamp>
+        <format property="timestamp" pattern="yyyy-MM-dd'T'HH:mm:ssXXX"/>
+    </tstamp>
+
     <target name="clean"
             description="Deletes all generated files">
         <delete dir="${build_dir}"/>
@@ -71,7 +75,7 @@
             description="Creates JAR packages">
         <mkdir dir="${jar_dir}"/>
 
-        <jar destfile="${jar_dir}/xz.jar">
+        <jar destfile="${jar_dir}/xz.jar" modificationtime="${timestamp}">
             <fileset dir="${classes_dir}" includes="org/tukaani/xz/**"/>
             <zipfileset prefix="META-INF/versions/9/" dir="${classes9_dir}"
                         unless:true="${java8only}"/>
@@ -95,55 +99,69 @@
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/TestAllocSpeed.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/TestAllocSpeed.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="TestAllocSpeed.class">
             <manifest>
                 <attribute name="Main-Class" value="TestAllocSpeed"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/XZEncDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/XZEncDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="XZEncDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="XZEncDemo"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/XZDecDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/XZDecDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="XZDecDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="XZDecDemo"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/XZSeekEncDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/XZSeekEncDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="XZSeekEncDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="XZSeekEncDemo"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/XZSeekDecDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/XZSeekDecDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="XZSeekDecDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="XZSeekDecDemo"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/LZMAEncDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/LZMAEncDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="LZMAEncDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="LZMAEncDemo"/>
                 <attribute name="Class-Path" value="xz.jar"/>
             </manifest>
         </jar>
 
-        <jar destfile="${jar_dir}/LZMADecDemo.jar" basedir="${classes_dir}"
+        <jar destfile="${jar_dir}/LZMADecDemo.jar"
+             modificationtime="${timestamp}"
+             basedir="${classes_dir}"
              includes="LZMADecDemo.class">
             <manifest>
                 <attribute name="Main-Class" value="LZMADecDemo"/>
@@ -176,9 +194,11 @@
               preservelastmodified="true" overwrite="true"/>
 
         <jar destfile="${maven_dir}/xz-${version}-javadoc.jar"
+             modificationtime="${timestamp}"
              basedir="${doc_dir}"/>
 
-        <jar destfile="${maven_dir}/xz-${version}-sources.jar">
+        <jar destfile="${maven_dir}/xz-${version}-sources.jar"
+             modificationtime="${timestamp}">
             <fileset dir="${src_dir}" includes="org/tukaani/xz/**"/>
             <zipfileset prefix="META-INF/versions/9/" dir="${src9_dir}"/>
         </jar>