round: Don't override undeclared system function on IRIX 6.5.

round: Don't override undeclared system function on IRIX 6.5.

* m4/check-math-lib.m4 (gl_CHECK_MATH_LIB): Accept an optional third
argument.
* m4/round.m4 (gl_FUNC_ROUND): Search for round() in the libraries
also when it is not declared. Set HAVE_ROUND. For replacement code,
test HAVE_ROUND, not HAVE_DECL_ROUND.
* modules/round (configure.ac): For replacement code, test HAVE_ROUND,
not HAVE_DECL_ROUND.
(Depends-on): Update conditions.
* modules/round-ieee (Depends-on): Update conditions.
* doc/posix-functions/round.texi: Mention the IRIX problem.