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>

helpers.c

@@ -91,3 +91,13 @@ char* strtok_r(char *str, const char *delim, char **nextp)
 }
 #endif
 
+/* There is no strnlen in DJGPP */
+#if defined(__DJGPP__)
+size_t strnlen(const char *str, size_t n)
+{
+	size_t i;
+	for (i = 0; i < n && str[i] != '\0'; i++)
+		;
+	return i;
+}
+#endif