This patch eliminates the need for providing "xjavac", which saves building a
package for the unmaintained "xerces-tools".

Taken from https://anonscm.debian.org/viewvc/pkg-java/trunk/libxerces2-java/debian/patches/01_xjavac_taskdef.patch?revision=14507

Index: b/build.xml
===================================================================
--- a/build.xml
+++ b/build.xml
@@ -39,7 +39,7 @@
   <property name="tools.dir" value="./tools"/>
 
   <!-- enable compilation under JDK 1.4 and above -->
-  <taskdef name="xjavac" classname="org.apache.xerces.util.XJavac">
+  <taskdef name="xjavac" classname="org.apache.tools.ant.taskdefs.Javac">
     <classpath>
       <pathelement location="${tools.dir}/bin/xjavac.jar"/>
     </classpath>
@@ -291,7 +291,7 @@
            source="${javac.source}"
            target="${javac.target}"
            classpath="${build.dir}/classes:${tools.dir}/${jar.apis}:${tools.dir}/${jar.resolver}:${tools.dir}/${jar.serializer}"
-           debug="${debug}"
+           debug="${debug}" nowarn="true"
            debuglevel="${debuglevel}"
            deprecation="${deprecation}"
            optimize="${optimize}"
@@ -359,7 +359,7 @@
            source="${javac.source}"
            target="${javac.target}"
            classpath="${build.dir}/classes:${tools.dir}/${jar.apis}"
-           debug="${debug}"
+           debug="${debug}" nowarn="true"
            debuglevel="${debuglevel}"
            includeAntRuntime="false"
            includeJavaRuntime="true"/>
@@ -379,7 +379,7 @@
            source="${javac.source}"
            target="${javac.target}"
            classpath="${tools.dir}/${jar.apis}:${build.dir}/classes:./tools/junit.jar"
-           debug="${debug}"
+           debug="${debug}" nowarn="true"
            debuglevel="${debuglevel}"
            includeAntRuntime="false"
            includeJavaRuntime="true"/>
earch'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=48d8176e006a6e7460b81c2386119181cf360ca5'>root</a>/<a href='/guix/log/.guix-channel?id=48d8176e006a6e7460b81c2386119181cf360ca5'>.guix-channel</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/.guix-channel?id=48d8176e006a6e7460b81c2386119181cf360ca5&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-06-17 19:32:43 +0200'>2020-06-17</span></td><td><a href='/guix/commit/.guix-channel?id=8b1f7c03d239ca703b56f2a6e5f228c79bc1857e'>.guix-channel: Add primary URL.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-16 16:10:47 +0200'>2020-06-16</span></td><td><a href='/guix/commit/.guix-channel?id=a941e8fe1f2ef47be2b30a54925febbc7dfb5389'>.guix-channel: Add 'keyring-reference'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-23 10:41:30 +0200'>2019-09-23</span></td><td><a href='/guix/commit/.guix-channel?id=dcc90d15581189dbc30e201db2b807273d6484f0'>Add '.guix-channel' file.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>