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"/>
or</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=c61e7ddd294a1e15df97b329112b92ecd9a9781a'>root</a>/<a href='/guix/log/gnu?id=c61e7ddd294a1e15df97b329112b92ecd9a9781a'>gnu</a>/<a href='/guix/log/gnu/packages?id=c61e7ddd294a1e15df97b329112b92ecd9a9781a'>packages</a>/<a href='/guix/log/gnu/packages/aux-files?id=c61e7ddd294a1e15df97b329112b92ecd9a9781a'>aux-files</a>/<a href='/guix/log/gnu/packages/aux-files/run-in-namespace.c?id=c61e7ddd294a1e15df97b329112b92ecd9a9781a'>run-in-namespace.c</a></div><div class='content'><table class='<tr><td><span title='2020-01-29 11:03:10 +0100'>2020-01-29</span></td><td><a href='/guix/commit/gnu/packages/hexedit.scm?id=602059e794182a035f3919c3eb88d7d2085e24df'>gnu: hexedit: Make F1 help more reliable.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/hexedit.scm (hexedit)[arguments](patch-man-path): New
phase.
[inputs]: Add MAN-DB.

Signed-off-by: Marius Bakke &lt;mbakke@fastmail.com&gt;
</span></span></td><td>Jakub Kądziołka</td></tr>
<tr><td><span title='2020-01-28 00:36:39 +0100'>2020-01-28</span></td><td><a href='/guix/commit/gnu/packages/hexedit.scm?id=7ee8acbb76bd440a8985a4b2a20b75521c6853ed'>gnu: Add bvi.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/hexedit.scm (bvi): New variable.

Signed-off-by: Marius Bakke &lt;mbakke@fastmail.com&gt;
</span></span></td><td>Jakub Kądziołka</td></tr>