configure: Add -Wno-char-subscripts.

Silences many warnings on systems with signed char where
the compiler cannot distinguish between char and int8_t.

configure

@@ -6509,6 +6509,7 @@ check_disable_warning -Wno-format-zero-length
 check_disable_warning -Wno-pointer-sign
 check_disable_warning -Wno-unused-const-variable
 check_disable_warning -Wno-bool-operation
+check_disable_warning -Wno-char-subscripts
 
 check_disable_warning_headers(){
     warning_flag=-W${1#-Wno-}