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