CMake with Ninja generator delets some 3rdparty source files

[dbs4261]

Checklist

• I have searched for similar issues.
• For Python issues, I have tested with the latest development wheel.
• I have checked the release documentation and the latest documentation (for main branch).

Steps to reproduce the issue

I first cloned Open3D by:

git clone https://github.com/isl-org/Open3D.git
cd Open3D

Then, I build Open3D (on Ubuntu 22.04) with:

mkdir cmake-build-debug
cmake -S . -B cmake-build-debug -G Ninja -DCMAKE_BUILD_TYPE=Debug -DGLIBCXX_USE_CXX11_ABI=ON -DBUILD_SHARED_LIBS=OFF -DBUILD_EXAMPLES=OFF -DBUILD_UNIT_TESTS=OFF -DBUILD_GUI=OFF -DWITH_SIMD=ON -DBUILD_WEBRTC=OFF -DUSE_BLAS=OFF -DOPEN3D_USE_ONEAPI_PACKAGES=ON -DUSE_SYSTEM_EIGEN3=ON -DUSE_SYSTEM_CURL=OFF -DBUILD_CURL_FROM_SOURCE=ON -DUSE_SYSTEM_OPENSSL=OFF -DBUILD_ISPC_MODULE=OFF
cmake --build cmake-build-debug --target clean

Error message

Result from `git status -s`

 D 3rdparty/glew/src/glew.c
 D 3rdparty/liblzf/liblzf/lzf_c.c
 D 3rdparty/liblzf/liblzf/lzf_d.c
 D 3rdparty/rply/rply/rply.c
 D 3rdparty/tinyfiledialogs/include/tinyfiledialogs/tinyfiledialogs.c

### Open3D, Python and System information

```markdown
- Operating system: Ubuntu 22.04
- Open3D version: 18.0
- System architecture: x86_64
- Is this a remote workstation?: no
- How did you install Open3D?: build from source
- Compiler version (if built from source): gnu 11.4.0

Additional information

No response

[dbs4261]

Looks like it stems from 3rdparty/find_dependencies.cmake:100. @stotko Git says you put the comment on the previous line in, can you explain what that changes and comment was fixing? I haven't been able to figure it out from my end. Why cant we just add the files in the add_library command?

[dbs4261]

Ok, Ive been playing with it a bit and I am starting to understand. When qhull is fetched with ExternalProject it is not downloaded until build time, so when we add_library at configure time, those files dont exist yet. I'll see what I can do to get around this.