Prioritize 'zlib from xcode sdk' flag correctly (#2791)

All include directories should go into CPPFLAGS.
XCode SDK was being added to CFLAGS instead
which caused old Tcl/Tk in the SDK to override a newer one in Homebrew.

plugins/python-build/bin/python-build

@@ -1616,7 +1616,7 @@ 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"
-    export CFLAGS="${CFLAGS:+$CFLAGS }-I${xc_sdk_path}/usr/include"
+    export CPPFLAGS="${CPPFLAGS:+$CPPFLAGS }-I${xc_sdk_path}/usr/include"
     if is_mac -ge 1100; then
       export LDFLAGS="${LDFLAGS:+$LDFLAGS }-L${xc_sdk_path}/usr/lib"
     fi