Do not use CMAKE_SOURCE_DIR.
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 #861.

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament