Fix confusing gmx_header_config.h include

Fix confusing gmx_header_config.h include

Easy to forget gmx_header_config.h because
historically only config.h is needed and
in master only config.h is needed, too.
Hid a bug in gmx_cpuid.c which didn't
do anything on Windows because the proper
define wasn't set.

Also fixes the compiler warnings in gmx_cpuid.c

Change-Id: I0ad9ca77bfe05551bdde82ab4e92b180c1e5963b