omp.h library not enclosed in #ifdef _OPENMP

Hello,
I'm using the submodule functionality of git to include your repo in a project (MeshLabJS), and I don't want to modify your code. Also, I don't want to use the omp library. Thus, I don't define the macro _OPENMP, but in the file MultiGridOctreeData.h, row #57, the library is not enclosed in #ifdef _OPENMP, unlike the functions in rows 65-66 on the same file, and compilation fails.