Test during configuration whether the linker accepts --no-as-needed.
Pass as_needed_flags to link commands.
Merge pull request #105 from yallop/detect_as_needed

Detect during configuration whether the linker accepts --no-as-needed
Include C objects in the native plugins (ctypes.cmxs etc.).
Merge pull request #81 from yallop/include-c-objects-in-native-plugin

Include C objects in the native plugins (ctypes.cmxs etc.).

Merging this, since the missing link flags were added in #105.  Fixes #79.
Make 'base' and 'base-install' targets.

The base targets just build and install ctypes and ctypes.top.
Remove obsolete function declaration.
Add a foreign-install target.
Remove another obsolete function declaration.