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>

flash.h

@@ -260,6 +260,9 @@ void tolower_string(char *str);
 #ifdef __MINGW32__
 char* strtok_r(char *str, const char *delim, char **nextp);
 #endif
+#if defined(__DJGPP__)
+size_t strnlen(const char *str, size_t n);
+#endif
 
 /* flashrom.c */
 extern const char flashrom_version[];