Use __attribute__((fallthrough)) on CLang

Fixes #427

Author: lb90

src/c99-flex.skl

@@ -52,7 +52,12 @@ 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]], [[
+	#ifdef __clang__
+	__attribute__((fallthrough));
+	#endif
+	/* 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 */]])

src/cpp-flex.skl

@@ -42,7 +42,12 @@ 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]], [[
+	#ifdef __clang__
+	__attribute__((fallthrough));
+	#endif
+	/* FALLTHROUGH */
+]])
 m4_define([[M4_HOOK_EOF_STATE_CASE_TERMINATE]], [[		yyterminate();
 ]])
 m4_define([[M4_HOOK_TAKE_YYTEXT]], [[YY_DO_BEFORE_ACTION; /* set up yytext */]])