fchdir: fix logic bugs Configuring with ac_cv_func_fchdir=no on a system that has fchdir and where open handles directories, just to test out the replacement capabilities, uncovered an m4 test bug and a link failure on rpl_fstat. * m4/fchdir.m4 (gl_FUNC_FCHDIR): Fix logic bug. * tests/test-fchdir.c (main): Enhance test. * lib/fchdir.c (rpl_fstat): Always provide if fchdir replacement is in use. Signed-off-by: Eric Blake <ebb9@byu.net>