Add comment about necessity of adding SDK paths to compiler flags

plugins/python-build/bin/python-build

@@ -1629,6 +1629,9 @@ use_xcode_sdk_zlib() {
   local xc_sdk_path="$(xcrun --show-sdk-path 2>/dev/null || true)"
   if [ -d "$xc_sdk_path" ]; then
     echo "python-build: use zlib from xcode sdk"
+    # Even though SDK's compiler uses the SDK dirs implicitly,
+    # CPython's setup.py has to have nonstandard paths specified explicitly
+    # to search for zlib.h in them
     export CPPFLAGS="${CPPFLAGS:+$CPPFLAGS }-I${xc_sdk_path}/usr/include"
     if is_mac -ge 1100; then
       export LDFLAGS="${LDFLAGS:+$LDFLAGS }-L${xc_sdk_path}/usr/lib"