exp2l-ieee: Work around test failure on OpenBSD 4.9 and IRIX 6.5.

exp2l-ieee: Work around test failure on OpenBSD 4.9 and IRIX 6.5.

* m4/exp2l-ieee.m4: New file.
* m4/exp2l.m4 (gl_FUNC_EXP2L): If gl_FUNC_EXP2L_IEEE is present,
test whether exp2l works with a NaN argument and with a negative
infinity argument. Replace it if not.
* lib/math.in.h (exp2l): Override if REPLACE_EXP2L is 1.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize REPLACE_EXP2L.
* modules/math (Makefile.am): Substitute REPLACE_EXP2L.
* modules/exp2l (configure.ac): Consider REPLACE_EXP2L.
(Depends-on): Update conditions.
* modules/exp2l-ieee (Files): Add m4/exp2l-ieee.m4.
(configure.ac): Invoke gl_FUNC_EXP2L_IEEE.
* doc/posix-functions/exp2l.texi: Mention the exp2l-ieee module.