Add libigl

[SimonDanisch]

No description provided.

[eschnett]

The error message CMake Error: The source directory "/workspace/srcdir/libigl/build/ " is weird. Note the trailing space.

You can try building in the main directory. For that, remove the mkdir build and cd build commands, and add the option -B build to cmake. Then build and install via

cmake --build build --parallel $(nproc)
cmake --install build