New module 'expm1-ieee'.

New module 'expm1-ieee'.

* modules/expm1-ieee: New file.
* m4/expm1-ieee.m4: New file.
* m4/expm1.m4 (gl_FUNC_EXPM1): If gl_FUNC_EXPM1_IEEE is present, test
whether expm1 works with a minus zero argument. Replace it if not.
* lib/math.in.h (expm1): Override if REPLACE_EXPM1 is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_EXPM1.
* modules/math (Makefile.am): Substitute REPLACE_EXPM1.
* modules/expm1 (configure.ac): Consider REPLACE_EXPM1.
(Depends-on): Update conditions.
* doc/posix-functions/expm1.texi: Mention the expm1-ieee module and the
AIX problem.