build: enable -Wcast-qual -Wwrite-strings CFLAGS

We're pretty careful about using const these days, so
enable these warnings to keep us that way.

bs/config.mk.default

@@ -10,7 +10,8 @@ CC = gcc
 CFLAGS = -O2 -Wall \
  -Wmissing-prototypes -Wextra -Wno-unused-parameter \
  -Wno-deprecated-declarations -Wmissing-prototypes \
- -Wdeclaration-after-statement -Wshadow
+ -Wdeclaration-after-statement -Wshadow -Wpointer-arith \
+ -Wstrict-prototypes -Wcast-qual -Wwrite-strings
 
 LDFLAGS = -Wl,-O1
 

configure.ac

@@ -32,6 +32,8 @@ then
 	MPD_CHECK_FLAG([-Wshadow])
 	MPD_CHECK_FLAG([-Wpointer-arith])
 	MPD_CHECK_FLAG([-Wstrict-prototypes])
+	MPD_CHECK_FLAG([-Wcast-qual])
+	MPD_CHECK_FLAG([-Wwrite-strings])
 fi
 
 if test -z "$prefix" || test "x$prefix" = xNONE; then