[PATCH v3 3/3] Makefile: Suppress `-Wimplicit-fallthrough` when compiling `lex.yy`

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

lex.yy.c is an auto generated C file. When compiling with clang-15, we
got the following warning:

```
      CC lex.yy.o
  lex.yy.c:1444:5: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
                                  case EOB_ACT_END_OF_FILE:
                                  ^
  lex.yy.c:1444:5: note: insert '__attribute__((fallthrough));' to silence this warning
                                  case EOB_ACT_END_OF_FILE:
                                  ^
                                  __attribute__((fallthrough));
  lex.yy.c:1444:5: note: insert 'break;' to avoid fall-through
                                  case EOB_ACT_END_OF_FILE:
                                  ^
                                  break;
  1 warning generated.
```

There is nothing we can do to fix lex.yy.c since it's an auto generated
file. Fix this by appending `-Wno-implicit-fallthrough` when compiling
this file if we have `-Wimplicit-fallthrough` flag enabled.

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index e670c1f2..1e15a69e 100644
--- a/Makefile
+++ b/Makefile
@@ -530,8 +530,12 @@ else
 	$(QUIET_LEX)$(LEX) $<
 endif
 
+ifneq (,$(findstring -Wimplicit-fallthrough,$(CFLAGS)))
+LEX_YY_CFLAGS := -Wno-implicit-fallthrough
+endif
+
 lex.yy.o: lex.yy.c y.tab.h
-	$(QUIET_CC)$(CC) -o $@ $(CFLAGS) $(CPPFLAGS) -c $<
+	$(QUIET_CC)$(CC) -o $@ $(CFLAGS) $(CPPFLAGS) $(LEX_YY_CFLAGS) -c $<
 
 y.tab.o: y.tab.c y.tab.h
 	$(QUIET_CC)$(CC) -o $@ $(CFLAGS) $(CPPFLAGS) -c $<
-- 
Ammar Faizi