Use __attribute__((fallthrough)) on CLang

Fixes #427

Author: lb90

src/c99-flex.skl

@@ -52,7 +52,10 @@ m4_define([[M4_HOOK_STATE_ENTRY_FORMAT]], [[	&yy_transition[$1],
 ]])
 m4_define([[M4_HOOK_NORMAL_STATE_CASE_ARM]], [[	case $1:]])
 m4_define([[M4_HOOK_EOF_STATE_CASE_ARM]], [[	case YY_STATE_EOF($1):]])
-m4_define([[M4_HOOK_EOF_STATE_CASE_FALLTHROUGH]], [[		/* FALLTHROUGH */]])
+m4_define([[M4_HOOK_EOF_STATE_CASE_FALLTHROUGH]], [[
+	yyfallthrough
+	/* FALLTHROUGH */
+]])
 m4_define([[M4_HOOK_EOF_STATE_CASE_TERMINATE]], [[		yyterminate();
 ]])
 m4_define([[M4_HOOK_TAKE_YYTEXT]], [[yy_do_before_action(yyscanner, yy_cp, yy_bp); /* set up yytext */]])
@@ -229,6 +232,22 @@ m4_ifdef( [[M4_YY_ALWAYS_INTERACTIVE]], ,
 #define yynoreturn
 #endif
 
+m4_changequote(<<, >>)
+
+#if defined(__has_c_attribute)
+#if __has_c_attribute(fallthrough)
+#define yyfallthrough [[fallthrough]];
+#endif
+#elif defined(__has_attribute)
+#if __has_attribute (fallthrough)
+#define yyfallthrough __attribute__((fallthrough));
+#endif
+#else
+#define yyfallthrough
+#endif
+
+m4_changequote([[, ]])
+
 /*
  * Anywhere other than C this won't be a thing,
  * because strings will have an associated length field.

src/cpp-flex.skl

@@ -42,7 +42,10 @@ m4_define([[M4_HOOK_STATE_ENTRY_FORMAT]], [[	&yy_transition[$1],
 ]])
 m4_define([[M4_HOOK_NORMAL_STATE_CASE_ARM]], [[	case $1:]])
 m4_define([[M4_HOOK_EOF_STATE_CASE_ARM]], [[	case YY_STATE_EOF($1):]])
-m4_define([[M4_HOOK_EOF_STATE_CASE_FALLTHROUGH]], [[		/* FALLTHROUGH */]])
+m4_define([[M4_HOOK_EOF_STATE_CASE_FALLTHROUGH]], [[
+	yyfallthrough
+	/* FALLTHROUGH */
+]])
 m4_define([[M4_HOOK_EOF_STATE_CASE_TERMINATE]], [[		yyterminate();
 ]])
 m4_define([[M4_HOOK_TAKE_YYTEXT]], [[YY_DO_BEFORE_ACTION; /* set up yytext */]])
@@ -301,6 +304,22 @@ m4preproc_include(`flexint_shared.h')
 #define yynoreturn
 #endif
 
+m4_changequote(<<, >>)
+
+#if defined(__has_cpp_attribute)
+#if __has_cpp_attribute(fallthrough)
+#define yyfallthrough [[fallthrough]];
+#endif
+#elif defined(__has_attribute)
+#if __has_attribute (fallthrough)
+#define yyfallthrough __attribute__((fallthrough));
+#endif
+#else
+#define yyfallthrough
+#endif
+
+m4_changequote([[, ]])
+
 m4_ifdef( [[M4_YY_IN_HEADER]],,[[m4_dnl
 /* Returned upon end-of-file. */
 #define YY_NULL 0