avoid bootclasspath warnings under java12

examples/nbproject/build-impl.xml

@@ -379,7 +379,11 @@ is divided into following sections:
                     <compilerarg value="-s"/>
                     <compilerarg path="@{apgeneratedsrcdir}"/>
                     <compilerarg line="${ap.proc.none.internal}"/>
-                    <customize/>
+                    <customize/>>
+                    <compilerarg value="-Xlint:all"/>
+                    <!-- http://ant.apache.org/manual/Tasks/javac.html#bootstrap -->
+                    <!-- https://stackoverflow.com/questions/4134803/ant-passing-compilerarg-into-javac -->
+                    <compilerarg value="-Xbootclasspath/p:${toString:lib.path.ref}"/>
                 </javac>
             </sequential>
         </macrodef>
@@ -423,6 +427,9 @@ 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>
@@ -458,6 +465,9 @@ 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>