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.