Use bootclasspath when building for java7

Note: this is only works correctly, if the property java7.home is set
Fixes #842

.travis/build-deploy.sh

@@ -26,7 +26,7 @@ VERSION=$(./mvnw -q -Dexec.executable="echo" -Dexec.args='${project.version}' --
 echo "Building PMD ${VERSION} on branch ${TRAVIS_BRANCH}"
 
 # TODO : Once we release PMD 6.0.0 and have a compatible PMD plugin, enable PMD once again
-MVN_BUILD_FLAGS="-B -V -Dpmd.skip=true"
+MVN_BUILD_FLAGS="-B -V -Djava7.home=/usr/lib/jvm/java-7-oracle -Dpmd.skip=true"
 
 if travis_isPullRequest; then
 

pmd-apex-jorje/pom.xml

@@ -14,6 +14,7 @@
   <properties>
     <config.basedir>${basedir}/../pmd-core</config.basedir>
     <java.version>8</java.version>
+    <java.home>${env.JAVA_HOME}</java.home>
     <apex.jorje.version>2017-11-17</apex.jorje.version>
   </properties>
 

pmd-apex/pom.xml

@@ -12,7 +12,9 @@
 
   <properties>
     <config.basedir>${basedir}/../pmd-core</config.basedir>
+
     <java.version>8</java.version>
+    <java.home>${env.JAVA_HOME}</java.home>
   </properties>
 
   <build>

pmd-java8/pom.xml

@@ -13,6 +13,7 @@
     <properties>
         <config.basedir>${basedir}/../pmd-core</config.basedir>
         <java.version>8</java.version>
+        <java.home>${env.JAVA_HOME}</java.home>
     </properties>
 
     <build>

pom.xml

@@ -255,11 +255,13 @@ Additionally it includes CPD, the copy-paste-detector. CPD finds duplicated code
     </issueManagement>
 
     <properties>
+        <java.version>7</java.version>
+        <java.home>${env.JAVA_HOME}</java.home>
+
         <javacc.version>5.0</javacc.version>
         <surefire.version>2.20.1</surefire.version>
         <checkstyle.version>2.17</checkstyle.version>
         <pmd.plugin.version>3.8</pmd.plugin.version>
-        <java.version>7</java.version>
         <ant.version>1.10.1</ant.version>
         <javadoc.plugin.version>3.0.0-M1</javadoc.plugin.version>
         <antlr.version>4.7</antlr.version>
@@ -341,6 +343,9 @@ Additionally it includes CPD, the copy-paste-detector. CPD finds duplicated code
                     <configuration>
                         <source>${java.version}</source>
                         <target>${java.version}</target>
+                        <compilerArguments>
+                            <bootclasspath>${java.home}/jre/lib/rt.jar</bootclasspath>
+                        </compilerArguments>
                     </configuration>
                 </plugin>
                 <plugin>
@@ -924,6 +929,18 @@ Additionally it includes CPD, the copy-paste-detector. CPD finds duplicated code
             </build>
         </profile>
 
+        <profile>
+            <id>jdk7-settings</id>
+            <activation>
+                <property>
+                    <name>java7.home</name>
+                </property>
+            </activation>
+            <properties>
+                <java.home>${java7.home}</java.home>
+            </properties>
+        </profile>
+
         <profile>
             <id>jdk8-modules</id>
             <activation>