* configure.in (AC_CHECK_HEADERS): Test for windows.h and dlfcn.h.
        * plugin.c: Guard include of dlfcn.h if HAVE_DLFCN_H is defined.
        Add windows.h header include if HAVE_WINDOWS_H is defined.
        (dlerror): New static function if windows variant is used instead
        of dlfcn.h.
        (dlclose): Likewise.
        (dlopen): Likewise.
        (dlsym): Likewise.
        * configure: Regenerated.
        * config.in: Regenerated.