expm1l: Don't override undeclared system function on IRIX 6.5.
* lib/math.in.h (expm1l): Test HAVE_DECL_EXPM1L, not HAVE_EXPM1L.
* m4/expm1l.m4 (gl_FUNC_EXPM1L): Don't assume expm1l() is declared when
it exists. Set HAVE_DECL_EXPM1L.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize HAVE_DECL_EXPM1L, not
HAVE_EXPM1L.
* modules/math (Makefile.am): Substitute HAVE_DECL_EXPM1L, not
HAVE_EXPM1L.
* doc/posix-functions/expm1l.texi: Mention missing declaration problem.