Add EXPORT_NAME property.

This allows for example, the buildsystem to use names like 'boost_any'
instead of the overly generic 'any', and still be able to generate
IMPORTED targets called 'boost::any'.

Tests/ExportImport/Export/testSharedLibDepends.cpp

@@ -4,5 +4,6 @@
 int TestSharedLibDepends::foo()
 {
   TestSharedLibRequired req;
-  return req.foo();
+  Renamed renamed;
+  return req.foo() + renamed.foo();
 }