Allow headers to be included with -Werror and -Wundef

Users of the library may enforce a stricter set of compiler warnings causing
their builds to fail due to warnings emitted from the headers.

include/uv-private/eio.h

@@ -251,7 +251,7 @@ struct eio_req
 
   eio_channel *channel; /* data used to direct poll callbacks arising from this req */
 
-#if __i386 || __amd64
+#if defined(__i386) || defined(__amd64)
   unsigned char cancelled;
 #else
   sig_atomic_t cancelled;

include/uv-private/ngx-queue.h

@@ -62,7 +62,7 @@ struct ngx_queue_s {
     (q)->prev
 
 
-#if (NGX_DEBUG)
+#if defined(NGX_DEBUG)
 
 #define ngx_queue_remove(x)                                                   \
     (x)->next->prev = (x)->prev;                                              \

include/uv-private/uv-unix.h

@@ -51,7 +51,7 @@
 # include <semaphore.h>
 #endif
 
-#if __sun
+#if defined(__sun)
 # include <sys/port.h>
 # include <port.h>
 #endif
@@ -103,7 +103,7 @@ struct uv__io_s {
 
 #define UV_REQ_TYPE_PRIVATE /* empty */
 
-#if __linux__
+#if defined(__linux__)
 # define UV_LOOP_PRIVATE_PLATFORM_FIELDS                                      \
   uv__io_t inotify_read_watcher;                                              \
   void* inotify_watchers;                                                     \