Replace all checks for 'linux' macro with '__linux__'

gcc -std=c99 does not define the former, and it's a horrible namespace
confusion anyway.

Signed-off-by: Julien Cristau <jcristau@debian.org>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

hw/xfree86/common/xf86Init.c

@@ -183,7 +183,7 @@ xf86PrintBanner(void)
             xf86ErrorFVerb(0, "Current Operating System: %s %s %s %s %s\n",
                            name.sysname, name.nodename, name.release,
                            name.version, name.machine);
-#ifdef linux
+#ifdef __linux__
             do {
                 char buf[80];
                 int fd = open("/proc/cmdline", O_RDONLY);