message: Do not use CMAKE_SOURCE_DIR.
  This is one of those CMake gotchas, we should always use
  CMAKE_CURRENT_SOURCE_DIR instead of CMAKE_SOURCE_DIR to allow
  Filament to exist as a subproject.
  In particular, this was causing the following build error when trying to
  build Filament as a nested project.
  combine-static-libs.sh: No such file or directory
  Fixes
diff: @@ -119,9 +119,9 @@ endforeach()
  # Run the combine-static-libs script to bundle the libraries together.
  set(FILAMAT_COMBINED_OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/libfilamat_combined.a")
  -set(COMBINE_SCRIPT "${CMAKE_SOURCE_DIR}/build/linux/combine-static-libs.sh")
  +set(COMBINE_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/../../build/linux/combine-static-libs.sh")
  if (WIN32)
  - set(COMBINE_SCRIPT "${CMAKE_SOURCE_DIR}/build/windows/combine-static-libs.bat")
  + set(COMBINE_SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/../../build/windows/combine-static-libs.bat")
  set(CMAKE_AR "lib.exe")
  endif()
  add_custom_command(
  