client.h: Added constant to identify the operating system

This constant is set to 1 if the operating system is Windows. Otherwise,
the constant is set to 0.

Signed-off-by: Adriano da Luz <al08@c3sl.ufpr.br>
Acked-by: Erik Alexandre Pucci <eap08@c3sl.ufpr.br>
Signed-off-by: Danilo K. S. Yorinori <danilok@c3sl.ufpr.br>