Fix the dyld error when running examples and tests on Mac.

Make sure that the install_name is set properly when linking the core library.

It would normally be possible to set this to <(PRODUCT_DIR) and let
gyp handle this, but this isn't supported (see gyp.XcodeSettings.GetInstallName)
so let qmake resolve the path.

This should be removed to use something relative to @loader_path that
would resolve this path to the Qt module libraries once we have proper
installation logic in place for the core library.

Change-Id: Id617afdf95c4aede6802e55b05deffe3218be82e
Reviewed-by: Andras Becsi <andras.becsi@digia.com>