log1p: Provide replacement for Minix and MSVC.

log1p: Provide replacement for Minix and MSVC.

* lib/math.in.h (log1p): New declaration.
* lib/log1p.c: New file.
* m4/log1p.m4: New file.
* m4/math_h.m4 (gl_MATH_H): Test whether log1p is declared.
(gl_MATH_H_DEFAULTS): Initialize GNULIB_LOG1P, HAVE_LOG1P.
* modules/math (Makefile.am): Substitute GNULIB_LOG1P, HAVE_LOG1P.
* modules/log1p (Files): Add lib/log1p.c, m4/log1p.m4.
(Depends-on): Add math, isnand, log, round.
(configure.ac): Invoke gl_FUNC_LOG1P. Arrange to compile replacement if
HAVE_LOG1P is 0.
* tests/test-math-c++.cc: Check the declaration of log1p.
* doc/posix-functions/log1p.texi: Mention the replacement.