[5/11] Native MSVC support (dashL-envvar)

[Peter Rosin]

The MSVC linker doesn't have a -L option, and doesn't understand posix
paths. Move the paths to an envvar (LINK) with a prefix (-LIBPATH:)

        * libltdl/m4/libtool.m4: Add new tag variables to specify
        to what variable (dashL_envvar) -L options should be moved,
        and what to prepend to the -L path (dashL_envvar_spec).
        * libltdl/config/ltmain.m4sh (func_dashL_to_envvar): New
        function that moves -L options to a variable.
        * libltdl/config/ltmain.m4sh (func_mode_link): Prior to
        linking, call above function if dashL_envvar is set.

dashL-envvar.patch
Description: Text document