* libltdl/ltdl.c: Move standard headers and preprocessor guards

* libltdl/ltdl.c: Move standard headers and preprocessor guards
from here...
* libltdl/lt__private.h: ...to here.  New file to declare ltdl's
internal interfaces.
* libltdl/ltdl.c: Include lt__private.h.
* m4/ltdl.m4 (AC_LIB_LTDL): Add lt_dirent to AC_LIBOBJ if all of
opendir, readdir and closedir are missing.
(AC_CHECK_HEADERS): Remove assert.h, ctype.h, errno.h, malloc.h,
stdio.h and stdlib.h; these headers are all available in standard
c89 environments and newer.
* libltdl/lt__private.h: Include them here unconditionally.
* libltdl/lt__dirent.c (opendir, readdir, closedir): New file.
Windows dirent emulation functions moved to here...
* libltdl/ltdl.c (opendir, readdir, closedir): ...from here.
* libltdl/lt__dirent.h: New file.  Rename the global symbols from
lt__dirent.c into the lt__ namespace so they don't clash with
other libraries.