logf-ieee: Work around test failure on NetBSD 5.1. * m4/logf-ieee.m4: New file. * m4/logf.m4 (gl_FUNC_LOGF): If gl_FUNC_LOGF_IEEE is present, test whether logf works with a negative argument. Replace it if not. * lib/logf.c (logf): For negative arguments, return NaN. * modules/logf-ieee (Files): Add m4/logf-ieee.m4. (configure.ac): Invoke gl_FUNC_LOGF_IEEE. * doc/posix-functions/logf.texi: Mention the logf-ieee module.