PATCH: Fix build on Linux with glibc-2.1

[MacBaine]

Hello everyone,

I just tried to build openttd on an older Debian Potato system with glibc-2.1 and kernel 2.2.19. Since this glibc doesn't define in_addr_t and has no getifaddr a trivial fix is needed (see attachment).

It introduces a new parameter LINUX_GLIBC21 to the Makefile and makes the network_core header recognize it.

Regards,
Jim