isnan() not found causes compilation error
System: Archlinux (Kernel Version 4.4) - GCC 5.3.0

Change-Id: Id0bb4555428896678437e8abf228f1c208f1e7a0
Reviewed-on: https://gerrit.libreoffice.org/22695Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>