roundf: Don't override undeclared system function on IRIX 6.5. * m4/roundf.m4 (gl_FUNC_ROUNDF): Search for roundf() in the libraries also when it is not declared. Set HAVE_ROUNDF. For replacement code, test HAVE_ROUNDF, not HAVE_DECL_ROUNDF. * modules/roundf (configure.ac): For replacement code, test HAVE_ROUNDF, not HAVE_DECL_ROUNDF. (Depends-on): Update conditions. * modules/roundf-ieee (Depends-on): Update conditions. * doc/posix-functions/roundf.texi: Mention the IRIX problem.