macOS: set mac_sdk_min = "10.15"

This fixes the error: `binary_buffer.o has version 11.0.0, which is newer than target minimum of 10.13.0`

Author: bblanchon

steps/05-configure.sh

@@ -43,6 +43,7 @@ mkdir -p "$BUILD"
       ;;
     mac)
       echo 'mac_deployment_target = "10.13.0"'
+      echo 'mac_sdk_min = "10.15"'
       echo "clang_use_chrome_plugins = false"
       ;;
     emscripten)