w32/include/dlfcn.h: New file.
w32/compat/posixfcn.c: Include dlfcn.h.
(dlopen, dlerror, dlsym) [MAKE_LOAD]: New functions, in support of
dynamic loading.
config.h.W32.template (MAKE_LOAD): Define.
load.c (load_object) [HAVE_DOS_PATHS]: Support backslashes and
drive letters in file names of dynamic objects.
It should be sufficient, now, to write a new version of the load_object()
function. That function won't be called unless we really want to load, so all
checking and parsing is taken care of before that.
Provide a simple API for loaded objects to interact with GNU make. I still
won't guarantee that this API won't change but it's much closer to something
that's supported and provides easy-to-use interfaces with a public header
file.
