Set -sourceLevel when precompiling modules

Without -sourceLevel, the gwtar files would depend on the JDK used to
compile GWT. We want GWT to be compatible with Java 6 JVMs so the gwtar
files must be built with -sourceLevel 1.6 whichever the JDK used to
produce the GWT artifacts.

Change-Id: I69250d554f66615aa2d99f44c612d5eba0d5a8b8

elemental/build.xml

diff --git a/elemental/build.xml b/elemental/build.xml
index d30ec28..7578e0e 100644
--- a/elemental/build.xml
+++ b/elemental/build.xml
@@ -48,6 +48,8 @@
             <jvmarg value="-Xmx512M" />
             <module />
             <arg value="-strict" />
+            <arg value="-sourceLevel" />
+            <arg value="${javac.source}" />
             <arg value="-out" />
             <arg value="${project.build}/bin" />
           </java>

user/build.xml

diff --git a/user/build.xml b/user/build.xml
index 8ef85b7..2295a92 100755
--- a/user/build.xml
+++ b/user/build.xml
@@ -804,6 +804,8 @@
             <jvmarg value="-Xmx512M" />
             <module />
             <arg value="-strict" />
+            <arg value="-sourceLevel" />
+            <arg value="${javac.source}" />
             <arg value="-out" />
             <arg value="${project.build}/bin" />
           </java>