expl: avoid incorrect expl(small_value) on OpenBSD 5.4
OpenBSD 5.4, expl() incorrectly returns 'nan' for small values.
Detect this and use gnulib's expl() in that case.
Discussed here:
http://marc.info/?t=
139159081700002&r=1&w=2
(Message from David Coppa on 2014-02-05 to openbsd-tech mailing list).
Fixed here (in revision 1.2):
http://www.openbsd.org/cgi-bin/cvsweb/src/lib/libm/src/ld128/s_floorl.c
* m4/expl.m4 (gl_FUNC_EXPL): Add a check for this condition.
* doc/posix-functions/expl.texi: Mention the workaround.