Compute a library path from the native software model

Previously we were repeating the library path twice, but that's not
good for long-term maintenance.  That being said, extracting this
information from the depths of the native software model seems *far*
more complex than it should be.  I had hoped for something nicer in
response to
<https://discuss.gradle.org/t/compute-wl-rpath-flag-suitable-for-native-shared-library/25278>,
but so far there's nothing.

com.ibm.wala.cast/build.gradle

@@ -69,11 +69,22 @@ model {
 				exportedHeaders.srcDirs = ["$cSourceDir/include"]
 				lib library: 'jdk'
 			}
-			binaries.all {
-				switch ("$targetPlatform.operatingSystem.name/$targetPlatform.architecture.name") {
-					case 'linux/x86-64':
-						// TODO: compute path on following line from 'jdk' library properties somehow
-						linker.args '-Wl,-rpath', "$currentJavaHome/jre/lib/amd64/server"
+			binaries {
+				withType(StaticLibraryBinarySpec) {
+					buildable = false
+				}
+				withType(SharedLibraryBinarySpec) {
+					switch ("$targetPlatform.operatingSystem.name/$targetPlatform.architecture.name") {
+						case 'linux/x86-64':
+							$
+							.repositories['libs']
+							.resolveLibrary('jdk')
+							.binaries
+							.withType(SharedLibraryBinary)
+							.findResults { it.sharedLibraryLinkFile }
+							*.parentFile
+							.each { linker.args "-Wl,-rpath=$it" }
+					}
 				}
 			}
 		}