cc-wrapper: fix bool handling for empty and zero values

Before the code would fail silently for zero values and with some output for
empties. We now currently handle both via defaulting value to zero and making
`let` return success error code when there's no syntax error.

pkgs/build-support/cc-wrapper/utils.sh

@@ -25,7 +25,11 @@ mangleVarBool() {
     for infix in "${role_infixes[@]}"; do
         local inputVar="${var/+/${infix}}"
         if [ -v "$inputVar" ]; then
-            let "${outputVar} |= ${!inputVar}"
+            # "1" in the end makes `let` return success error code when
+            # expression itself evaluates to zero.
+            # We don't use `|| true` because that would silence actual
+            # syntax errors from bad variable values.
+            let "${outputVar} |= ${!inputVar:-0}" "1"
         fi
     done
 }