New module 'exp2l'.

New module 'exp2l'.

* lib/math.in.h (exp2l): New declaration.
* lib/exp2l.c: New file.
* lib/expl-table.c: New file, extracted from lib/expl.c.
* lib/expl.c (gl_expl_table): New declaration.
(expl): Remove expl_table. Update reference.
* m4/exp2l.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether exp2l is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_EXP2L, HAVE_DECL_EXP2L.
* modules/math (Makefile.am): Substitute GNULIB_EXP2L, HAVE_DECL_EXP2L.
* modules/exp2l: New file.
* modules/expl (Files): Add lib/expl-table.c.
(configure.ac): Compile also expl-table.c.
* tests/test-math-c++.cc: Check the declaration of exp2l.
* doc/posix-functions/exp2l.texi: Mention the new module and the IRIX
problem.