Changes

Allow linking empty .cmxa files on MSVC

MSVC .lib format doesn't support having no .obj files in the library, so
ocamlopt -o foo.cmxa -a generates foo.cmxa but not foo.lib (there's no
error from the Microsoft Linker). The resulting foo.cmxa is unlinkable,
since OCaml passes foo.lib on to the linker.

This patch relaxes the requirement for foo.lib if the .cmxa contains no
units.
 (commit: 793cd86)

Test for linking archives with no modules
 (commit: d732593)

Don't call the archiver/librarian for empty .cmxa
 (commit: e6ab329)