stdenv: fix prefixKey for structuredAttrs (#470397)

pkgs/by-name/er/eresi/package.nix

@@ -84,6 +84,8 @@ stdenv.mkDerivation rec {
 
   installTargets = lib.singleton "install" ++ lib.optional stdenv.hostPlatform.is64bit "install64";
 
+  __structuredAttrs = true;
+
   meta = {
     description = "ERESI Reverse Engineering Software Interface";
     license = lib.licenses.gpl2Only;

pkgs/stdenv/generic/setup.sh

@@ -1455,7 +1455,16 @@ configurePhase() {
     fi
 
     if [[ -z "${dontAddPrefix:-}" && -n "$prefix" ]]; then
-        prependToVar configureFlags "${prefixKey:---prefix=}$prefix"
+        # For __structuredAttrs: if prefixKey ends in a space,
+        # we need to add the prefixKey and the prefix as separate entries,
+        # and since we prepend, we do it in reverse order.
+        local -r prefixKeyOrDefault="${prefixKey:---prefix=}"
+        if [ "${prefixKeyOrDefault: -1}" = " " ]; then
+            prependToVar configureFlags "$prefix"
+            prependToVar configureFlags "${prefixKeyOrDefault::-1}"
+        else
+            prependToVar configureFlags "$prefixKeyOrDefault$prefix"
+        fi
     fi
 
     if [[ -f "$configureScript" ]]; then