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"/>
' value=''/>
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</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=4d1e0f902236c26afb96373dcf57f3ff70638de4'>root</a>/<a href='/guix/log/gnu?id=4d1e0f902236c26afb96373dcf57f3ff70638de4'>gnu</a>/<a href='/guix/log/gnu/system?id=4d1e0f902236c26afb96373dcf57f3ff70638de4'>system</a>/<a href='/guix/log/gnu/system/examples?id=4d1e0f902236c26afb96373dcf57f3ff70638de4'>examples</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/gnu/system/examples?id=4d1e0f902236c26afb96373dcf57f3ff70638de4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-04-02 11:48:25 +0200'>2020-04-02</span></td><td><a href='/guix/commit/gnu/system/examples?id=c7af96871737ffa0e2be879e200573bc60603c8c'>system: Add xterm to lightweight-desktop example.</a><span class='msg-avail'>...<span class='msg-tooltip'>That makes ratpoison more usable, for instance.

* gnu/system/examples/lightweight-desktop.tmpl (packages): Add XTERM.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-03-22 20:55:37 -0700'>2020-03-22</span></td><td><a href='/guix/commit/gnu/system/examples?id=37f8c50cdc2e99189c74999319f3a1854d419546'>gnu: linux-libre-arm-veyron: Deprecate package.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/linux (linux-libre-arm-veyron): Mark as deprecated package.
  (kernel-config-veyron): Remove obsolete function.
* gnu/system/examples/asus-c201.tmpl: Update to use linux-libre-arm-generic.
* gnu/packages/aux-files/linux-libre/5.4-arm-veyron.conf: Delete file.
* Makefile.am (AUX_FILES): Update accordingly.
</span></span></td><td>Vagrant Cascadian</td></tr>
<tr><td><span title='2019-12-07 00:59:56 +0100'>2019-12-07</span></td><td><a href='/guix/commit/gnu/system/examples?id=362bcdb1b076c8c46f71781add56dfbe532736dc'>system: Remove custom 'config-file service from "vm-image.tmpl".</a><span class='msg-avail'>...<span class='msg-tooltip'>This example is broken since commit
99c45877a984dd0148151b2e304afef6fb04f1a5 (wrong file name passed to
'local-file'), and its functionality is superseded by the upcoming
'provenance-service-type'.

* gnu/system/examples/vm-image.tmpl (this-file): Remove.
&lt;services&gt;: Remove 'config-file service.
</span></span></td><td>Ludovic Courtès</td></tr>