make-wrapper.sh: add an --unset argument

`--set FOO ""` is not strictly equivalent to `--unset FOO`. In the former case
the environment variable still exists with an empty string as a value.

pkgs/build-support/setup-hooks/make-wrapper.sh

@@ -19,6 +19,12 @@ makeWrapper() {
             echo "export $varName=$value" >> $wrapper
         fi
 
+        if test "$p" = "--unset"; then
+            varName=${params[$((n + 1))]}
+            n=$((n + 1))
+            echo "unset $varName" >> "$wrapper"
+        fi
+
         if test "$p" = "--run"; then
             command=${params[$((n + 1))]}
             n=$((n + 1))