Currently, on setups where the project dir is on a different file system as the system temporary directory, a temporary directory is instead created on the project dir. This is not very nice.
With this change, we still create temporary files in the system temporary directory but copy instead of move files from and to the temporary directory, if necessary, which solves the problems.
When running the make file with a number of jobs, the following warning was generated by the make process used to parse the dependency makefile generated by OpenSCAD:
make[1]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
With this change, the variables MAKELEVEL and MAKEFLAGS are removed from the environment before starting the child make process.
