log-ieee: Work around test failure on NetBSD 5.1 and Solaris 11.

log-ieee: Work around test failure on NetBSD 5.1 and Solaris 11.

* m4/log-ieee.m4: New file.
* m4/log.m4 (gl_FUNC_LOG): If gl_FUNC_LOG_IEEE is present, test whether
log works with a negative argument. Replace it if not.
* lib/log.c (log): For negative arguments, return NaN.
* modules/log-ieee (Files): Add m4/log-ieee.m4.
(configure.ac): Invoke gl_FUNC_LOG_IEEE.
* doc/posix-functions/log.texi: Mention the log-ieee module.