UseSWIG: Fall back to empty PREFIX for unknown languages

Teach `SWIG_ADD_LIBRARY` to fall back to an empty library prefix for
languages we don't explicitly define already.

Suggested-by: Alan W. Irwin <irwin@beluga.phys.uvic.ca>

Modules/UseSWIG.cmake

@@ -326,6 +326,9 @@ macro(SWIG_ADD_LIBRARY name)
     if (APPLE)
       set_target_properties (${SWIG_MODULE_${name}_REAL_NAME} PROPERTIES SUFFIX ".bundle")
     endif ()
+  else()
+    # assume empty prefix because we expect the module to be dynamically loaded
+    set_target_properties (${SWIG_MODULE_${name}_REAL_NAME} PROPERTIES PREFIX "")
   endif ()
 endmacro()