GlueGen (Compile Time): Add 'CStruct' Annotation Processor (APT) to 'gluegen' C Structs on-the-fly (2-pass actually).

Convenient annotation processing (APT) hooked to 'javac' (1.6) via gluegen.jar META-INF
service provider 'javax.annotation.processing.Processor' -> 'com.jogamp.gluegen.structgen.CStructAnnotationProcessor'.

Am implicit APT / JAVAC would be possible, however - to have a proper process
the unit test utilizes an explicit 2 pass run:

        <!-- Annotation Processor Only - First -->
        <javac destdir="${build_t.java}">
            <classpath refid="junit.compile.classpath"/>
            <compilerarg value="-proc:only"/>
            <compilerarg value="-J-Djogamp.gluegen.structgen.debug"/>
            <compilerarg value="-J-Djogamp.gluegen.structgen.output=${build_t.gen}/classes"/>
            <src path="${test.base.dir}/com/jogamp/gluegen/test/junit/structgen"/>
        </javac>

        <!-- Javac Only - Second -->
        <javac destdir="${build_t.java}">
            <classpath refid="junit.compile.classpath"/>
            <compilerarg value="-proc:none"/>
            <src path="${test.base.dir}"/>
            <src path="${build_t.gen}" />
        </javac>

Original code from Michael Bien's 'superglue' git://github.com/mbien/superglue.git,
finally merged to GlueGen (as once intended).

Note: The APT javac pass requires to use 'gluegen.jar' instead of 'gluegen-rt.jar' !
      The 2-pass process also alows using the runtime gluegen-rt.jar and hence ensures
      clean namespace check at compilation.