avoid bootclasspath warnings under java12

assignments/nbproject/build-impl.xml

@@ -383,6 +383,9 @@ is divided into following sections:
                     <compilerarg path="@{apgeneratedsrcdir}"/>
                     <compilerarg line="${ap.proc.none.internal}"/>
                     <customize/>
+                    <!-- http://ant.apache.org/manual/Tasks/javac.html#bootstrap -->
+                    <!-- https://stackoverflow.com/questions/4134803/ant-passing-compilerarg-into-javac -->
+                    <compilerarg value="-Xbootclasspath/p:${toString:lib.path.ref}"/>
                 </javac>
             </sequential>
         </macrodef>
@@ -426,6 +429,9 @@ is divided into following sections:
                     <compilerarg path="@{apgeneratedsrcdir}"/>
                     <compilerarg line="${ap.proc.none.internal}"/>
                     <customize/>
+                    <!-- http://ant.apache.org/manual/Tasks/javac.html#bootstrap -->
+                    <!-- https://stackoverflow.com/questions/4134803/ant-passing-compilerarg-into-javac -->
+                    <compilerarg value="-Xbootclasspath/p:${toString:lib.path.ref}"/>
                 </javac>
             </sequential>
         </macrodef>
@@ -461,6 +467,9 @@ is divided into following sections:
                     <compilerarg line="${javac.profile.cmd.line.arg}"/>
                     <compilerarg line="${javac.compilerargs}"/>
                     <customize/>
+                    <!-- http://ant.apache.org/manual/Tasks/javac.html#bootstrap -->
+                    <!-- https://stackoverflow.com/questions/4134803/ant-passing-compilerarg-into-javac -->
+                    <compilerarg value="-Xbootclasspath/p:${toString:lib.path.ref}"/>
                 </javac>
             </sequential>
         </macrodef>