dup2: Move AC_LIBOBJ invocation to module description. * m4/dup2.m4 (gl_REPLACE_DUP2): Remove macro. (gl_FUNC_DUP2): Instead of gl_REPLACE_DUP2, just set REPLACE_DUP2 to 1. Don't invoke AC_LIBOBJ. * modules/dup2 (configure.ac): Invoke AC_LIBOBJ.