Older kernel versions do not export __u64 symbol when compiled in ansi mode (i.e. __STRICT_ANSI__ symbol defined)
resulting in errors like:

	.../include/linux/rtnetlink.h: error: unknown type name '__u64'

Fix it by changing the language standard used.

--- configure
+++ configure
@@ -6294,7 +6294,7 @@
 # Set -std=c99 unless user already specified a -std=
 case "${CFLAGS}" in
   *-std=*) ;;
-  *)       CFLAGS="${CFLAGS} -std=c99" ;;
+  *)       CFLAGS="${CFLAGS} -std=gnu99" ;;
 esac
 
 #