9c: add more clang warning silencers, use signed chars

R=rsc
http://codereview.appspot.com/6741053

bin/9c

@@ -15,6 +15,7 @@ usegcc()
 		-Wno-sign-compare \
 		-Wno-unknown-pragmas \
 		-fno-omit-frame-pointer \
+		-fsigned-char \
 	"
 	# want to put -fno-optimize-sibling-calls here but
 	# that option only works with gcc3+ it seems
@@ -56,11 +57,15 @@ useclang()
 		-O2 \
 		-c \
 		-Wall \
-		-Wno-comment \
-		-Wno-empty-body \
 		-Wno-parentheses \
+		-Wno-missing-braces \
+		-Wno-switch \
+		-Wno-comment \
+		-Wno-sign-compare \
 		-Wno-unknown-pragmas \
+		-Wno-empty-body \
 		-Wno-unused-value \
+		-fsigned-char \
 	"
 	cflags="$ngflags -g"
 }