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"/>
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</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=b380fbca8859f7ba0f41703a970ddd26b4590eb4'>root</a>/<a href='/guix/log/doc?id=b380fbca8859f7ba0f41703a970ddd26b4590eb4'>doc</a>/<a href='/guix/log/doc/environment-gdb.scm?id=b380fbca8859f7ba0f41703a970ddd26b4590eb4'>environment-gdb.scm</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/doc/environment-gdb.scm?id=b380fbca8859f7ba0f41703a970ddd26b4590eb4&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-01-14 22:39:24 +0100'>2022-01-14</span></td><td><a href='/guix/commit/doc/environment-gdb.scm?id=b0aab0f255556c8c5027e3ad11bfc7826bb1f609'>doc: Update "guix shell -D -f" example to new input style.</a><span class='msg-avail'>...<span class='msg-tooltip'>* doc/environment-gdb.scm: Change to use 'modify-inputs'.
</span></span></td><td>Ludovic Courtès</td></tr>