Makefile: Disable warning for Bison-generated source

Bison-3.4.2 together with llvm 15.0.4 produce the warning

    parser.tab.c:1078:9: warning: variable 'yynerrs' set but not used

Disable this warning for the parser.tab.c compilation.

Makefile

@@ -361,6 +361,14 @@ ppidcache.h += ppidcache.h
 
 ########################################################################
 
+## parser.tab.o
+
+# Disable "variable 'yynerrs' set but not used" diagnostic which
+# appears with bison-3.4.2 + llvm 15.0.4
+
+parser.tab.o: parser.tab.c parser.tab.h
+	$(call quiet-command,${CC} ${CFLAGS} -Wno-unused-but-set-variable -o $@ -c $<,"     CC $@")
+
 ### mx_getopt.o --------------------------------------------------------
 
 mx_getopt.o: $(mx_getopt.h)