maint: avoid implicit-fallthrough warnings from GCC 7

Avoid the new warnings GCC 7 gives with the
"-Werror=implicit-fallthrough=" compiler option.
Since version 7, GCC requires fallthrough switch/cases to be
marked by the appropriate attribute.

* lib/system.h: Add new header file defining the FALLTHROUGH macro.
* lib/Makefile.am (libfind_a_SOURCES): Reference it.
* find/print.c (do_fprintf): Use the new macro.
* xargs/xargs.c (read_line): Likewise.
(xargs_do_exec): For error(3), GCC 7 doesn't know that the function
does not return, and would complain about falling through into the
following case.  Temporarily add an abort call - this will be fixed
with another commit.

find/print.c

@@ -44,6 +44,7 @@
 #include "xalloc.h"
 
 /* find-specific headers. */
+#include "system.h"
 #include "defs.h"
 #include "print.h"
 
@@ -951,13 +952,8 @@ do_fprintf (struct format_val *dest,
                 checked_fprintf (dest, segment->text, g->gr_name);
                 break;
               }
-            else
-              {
-                /* Do nothing. */
-                /*FALLTHROUGH*/
-              }
           }
-          /*FALLTHROUGH*/ /*...sometimes, so 'G' case.*/
+          FALLTHROUGH; /*...sometimes, so 'G' case.*/
 
         case 'G':               /* GID number */
           /* UNTRUSTED, probably unexploitable */
@@ -1165,9 +1161,8 @@ do_fprintf (struct format_val *dest,
                 checked_fprintf (dest, segment->text, p->pw_name);
                 break;
               }
-            /* else fallthru */
           }
-          /* FALLTHROUGH*/ /* .. to case U */
+          FALLTHROUGH; /* .. to case U */
 
         case 'U':               /* UID number */
           /* UNTRUSTED, probably unexploitable */

lib/Makefile.am

@@ -56,7 +56,8 @@ libfind_a_SOURCES = \
 	splitstring.c \
 	splitstring.h \
 	bugreports.c \
-	bugreports.h
+	bugreports.h \
+	system.h
 
 EXTRA_DIST = unused-result.h check-regexprops.sh
 SUFFIXES =

lib/system.h

@@ -0,0 +1,43 @@
+/* system.h -- system-dependent definitions for findutils
+
+   Copyright (C) 2017 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+#if !defined SYSTEM_H
+# define SYSTEM_H
+
+/* FALLTHROUGH
+ * Since GCC7, the "-Werror=implicit-fallthrough=" option requires
+ * fallthrough cases to be marked as such via:
+ *     __attribute__ ((__fallthrough__))
+ * Usage:
+ *   switch (c)
+ *     {
+ *     case 1:
+ *       doOne();
+ *       FALLTHROUGH;
+ *     case 2:
+ *     ...
+ *     }
+ */
+#ifndef FALLTHROUGH
+# if __GNUC__ < 7
+#  define FALLTHROUGH ((void) 0)
+# else
+#  define FALLTHROUGH __attribute__ ((__fallthrough__))
+# endif
+#endif
+
+#endif /* SYSTEM_H */

xargs/xargs.c

@@ -67,6 +67,7 @@
 #include "bugreports.h"
 #include "findutils-version.h"
 #include "gcc-function-attributes.h"
+#include "system.h"
 
 #if ENABLE_NLS
 # include <libintl.h>
@@ -904,7 +905,7 @@ read_line (void)
 	  if (ISSPACE (c))
 	    continue;
 	  state = NORM;
-	  /* aaahhhh....  */
+	  FALLTHROUGH;  /* aaahhhh....  */
 
 	case NORM:
 	  if (c == '\n')
@@ -1277,6 +1278,9 @@ xargs_do_exec (struct buildcmd_control *ctl, void *usercontext, int argc, char *
 	{
 	case -1:
 	  error (EXIT_FAILURE, errno, _("cannot fork"));
+	  /* error(EXIT_FAILURE, ...) does not return, but tell GCC 7 that
+	     we don't fall through here; fixed with another commit.  */
+	  abort();
 
 	case 0:		/* Child.  */
 	  {