Set paths of papi dependencies programmatically

If it now works, it will be the end of years of struggle to get these
damn library.path set!

build.gradle

@@ -63,7 +63,8 @@ class PAPIPlugin implements Plugin<Project> {
 
     // We want to define some properties...
     void apply(Project project) {
-        def library_path = "$papi_path:../papi-java/".toString()
+        def papi_java_path = "${project.rootProject.projectDir}/papi-java"
+        def library_path = "$papi_path:$papi_java_path".toString()
 
         project.ext.papi_library_path = library_path
     }