xcbuild: try to fix sdk not found

This is an attempt to get rid of the issues with sdk not found in
xcbuild when building pinentry_mac.

pkgs/development/tools/xcbuild/platform.nix

@@ -241,6 +241,6 @@ stdenv.mkDerivation {
 
     mkdir -p $out/Developer/SDKs/
     cd $out/Developer/SDKs/
-    ln -s ${sdk}
+    ln -s ${sdk} macosx10.10.sdk
   '';
 }

pkgs/development/tools/xcbuild/wrapper.nix

@@ -60,9 +60,13 @@ stdenv.mkDerivation {
       --add-flags "DERIVED_DATA_DIR=." \
       --set DEVELOPER_DIR "$out"
     wrapProgram $out/bin/xcrun \
+      --add-flags "-sdk macosx10.10" \
       --set DEVELOPER_DIR "$out"
     wrapProgram $out/bin/xcode-select \
       --set DEVELOPER_DIR "$out"
+
+    mkdir -p $out/usr/bin/
+    ln -s $out/bin/xcrun $out/usr/bin/xcrun
   '';
 
   inherit (xcbuild) meta;