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