Port --enable-gcc-warnings to GCC 6.2.1

Backport from master.
* src/conf_post.h (GNUC_PREREQ): New macro.
* src/keyboard.c: Use it to work around GCC bug 54561.
* src/process.c (would_block): New function.
(server_accept_connection, wait_reading_process_output, send_process):
Use it.

src/conf_post.h

@@ -34,6 +34,17 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 
 #include <stdbool.h>
 
+/* GNUC_PREREQ (V, W, X) is true if this is GNU C version V.W.X or later.
+   It can be used in a preprocessor expression.  */
+#ifndef __GNUC_MINOR__
+# define GNUC_PREREQ(v, w, x) false
+#elif ! defined __GNUC_PATCHLEVEL__
+# define GNUC_PREREQ(v, w, x) ((v) < __GNUC__ + ((w) <= __GNUC_MINOR__))
+#else
+# define GNUC_PREREQ(v, w, x) \
+    ((v) < __GNUC__ + ((w) <= __GNUC_MINOR__ + ((x) <= __GNUC_PATCHLEVEL__)))
+#endif
+
 /* The type of bool bitfields.  Needed to compile Objective-C with
    standard GCC.  It was also needed to port to pre-C99 compilers,
    although we don't care about that any more.  */

src/keyboard.c

@@ -70,6 +70,11 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include TERM_HEADER
 #endif /* HAVE_WINDOW_SYSTEM */
 
+/* Work around GCC bug 54561.  */
+#if GNUC_PREREQ (4, 3, 0)
+# pragma GCC diagnostic ignored "-Wclobbered"
+#endif
+
 /* Variables for blockinput.h:  */
 
 /* Positive if interrupt input is blocked right now.  */

src/process.c

@@ -151,6 +151,18 @@ bool inhibit_sentinels;
 # define SOCK_CLOEXEC 0
 #endif
 
+/* True if ERRNUM represents an error where the system call would
+   block if a blocking variant were used.  */
+static bool
+would_block (int errnum)
+{
+#ifdef EWOULDBLOCK
+  if (EWOULDBLOCK != EAGAIN && errnum == EWOULDBLOCK)
+    return true;
+#endif
+  return errnum == EAGAIN;
+}
+
 #ifndef HAVE_ACCEPT4
 
 /* Emulate GNU/Linux accept4 and socket well enough for this module.  */
@@ -4262,15 +4274,7 @@ server_accept_connection (Lisp_Object server, int channel)
   if (s < 0)
     {
       int code = errno;
-
+      if (!would_block (code) && !NILP (ps->log))
-      if (code == EAGAIN)
-	return;
-#ifdef EWOULDBLOCK
-      if (code == EWOULDBLOCK)
-	return;
-#endif
-
-      if (!NILP (ps->log))
 	call3 (ps->log, server, Qnil,
 	       concat3 (build_string ("accept failed with code"),
 			Fnumber_to_string (make_number (code)),
@@ -4687,12 +4691,8 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 		  int nread = read_process_output (proc, wait_proc->infd);
 		  if (nread < 0)
 		    {
-		    if (errno == EIO || errno == EAGAIN)
+		      if (errno == EIO || would_block (errno))
-		      break;
+			break;
-#ifdef EWOULDBLOCK
-		    if (errno == EWOULDBLOCK)
-		      break;
-#endif
 		    }
 		  else
 		    {
@@ -5073,11 +5073,7 @@ wait_reading_process_output (intmax_t time_limit, int nsecs, int read_kbd,
 		  if (do_display)
 		    redisplay_preserve_echo_area (12);
 		}
-#ifdef EWOULDBLOCK
+	      else if (nread == -1 && would_block (errno))
-	      else if (nread == -1 && errno == EWOULDBLOCK)
-		;
-#endif
-	      else if (nread == -1 && errno == EAGAIN)
 		;
 #ifdef WINDOWSNT
 	      /* FIXME: Is this special case still needed?  */
@@ -5801,11 +5797,7 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
 
 	  if (rv < 0)
 	    {
-	      if (errno == EAGAIN
+	      if (would_block (errno))
-#ifdef EWOULDBLOCK
-		  || errno == EWOULDBLOCK
-#endif
-		  )
 		/* Buffer is full.  Wait, accepting input;
 		   that may allow the program
 		   to finish doing output and read more.  */