* lib/math.in.h (exp2): New declaration.
* lib/exp2.c: New file.
* m4/exp2.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether exp2 is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_EXP2, HAVE_DECL_EXP2,
REPLACE_EXP2.
* modules/math (Makefile.am): Substitute GNULIB_EXP2, HAVE_DECL_EXP2,
REPLACE_EXP2.
* modules/exp2: New file.
* tests/test-math-c++.cc: Check the declaration of exp2.
* doc/posix-functions/exp2.texi: Mention the new module and the IRIX
and OpenBSD problems.