Fix export no registry (#27)

Using `INTERNAL` will force it to off and there is no way to enable it. I have the same fix in the GridTools branch for gridtools core.

Author: havogt

CMakeLists.txt

@@ -5,8 +5,9 @@ file(STRINGS "version.txt" __CPP_BINDGEN_VERSION)
 project(cpp_bindgen VERSION ${__CPP_BINDGEN_VERSION} LANGUAGES CXX)
 unset(__CPP_BINDGEN_VERSION)
 
-# Switch default of NO_PACKAGE_REGISTRY. TODO: Can be removed after CMake 3.15
-set(CMAKE_EXPORT_NO_PACKAGE_REGISTRY ON CACHE INTERNAL "")
+# Switch default of NO_PACKAGE_REGISTRY. TODO: Can be removed with CMake 3.15+
+set(CMAKE_EXPORT_NO_PACKAGE_REGISTRY ON CACHE BOOL "")
+mark_as_advanced(CMAKE_EXPORT_NO_PACKAGE_REGISTRY)
 
 include(CTest)