Extra help for finding "cast" native library on macOS

Unlike Linux, macOS has no "RPATH" facility for embedding additional
search paths within shared libraries.  Instead, we need to set
"DYLD_LIBRARY_PATH" appropriately in the environment.  This
environment variable is the macOS analogue of "LD_LIBRARY_PATH" on
Linux.

Note that adding the required path to the "java.library.path" system
property will *not* work.  This property only affects where the JVM
looks for shared objects that it is loading directly.  This property
does not influence the search for other, transitively-required shared
objects.

Fixes #3.

com.ibm.wala.cast.test/build.gradle

@@ -97,9 +97,9 @@ model {
 		def lib = linkXlator_testSharedLibrary
 		dependsOn lib
 		systemProperty 'java.library.path', lib.destinationDir
+
+		// TODO: compute path on following line from 'cast' library properties somehow
+		def castLibDir = "${project(':com.ibm.wala.cast').buildDir}/libs/cast/shared"
+		environment 'DYLD_LIBRARY_PATH', castLibDir
 	}
 }
-
-// https://github.com/liblit/WALA/issues/3
-if (System.getProperty('os.name') == 'Mac OS X')
-	test.exclude '**/TestNativeTranslator.class'