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

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

* lib/math.in.h (remainderl): Test HAVE_DECL_REMAINDERL, not
HAVE_REMAINDERL.
* m4/remainderl.m4 (gl_FUNC_REMAINDERL): Don't assume remainderl() is
declared when it exists. Set HAVE_DECL_REMAINDERL.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize HAVE_DECL_REMAINDERL,
not HAVE_REMAINDERL.
* modules/math (Makefile.am): Substitute HAVE_DECL_REMAINDERL, not
HAVE_REMAINDERL.
* doc/posix-functions/remainderl.texi: Mention missing declaration
problem.