Make strnlen() visible in old versions of glibc

Strnlen() is in POSIX 2008 but was a GNU extension up to glibc 2.10
requiring to define _GNU_SOURCE. This fixes compilation on CentOS 4.9.
Also, move our implementation of strnlen() that was added to support
DJGPP to helpers.c.

Corresponding to flashrom svn r1878.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
3 files changed