* lib/math.in.h (atanf): New declaration.
* lib/atanf.c: New file.
* m4/atanf.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether atanf is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_ATANF, HAVE_ATANF.
* modules/math (Makefile.am): Substitute GNULIB_ATANF, HAVE_ATANF.
* modules/atanf: New file.
* tests/test-math-c++.cc: Check the declaration of atanf.
* doc/posix-functions/atanf.texi: Mention the new module.