If no drawing commands are called in an Asymptote script, no PDF output is created and no error is generated. This confuses the build process, which expect a target to exist after its commands were called.
With this change, the Python wrapper checks whether a PDF was actually written and throws an error otherwise.
This catches the OSError thrown by the subprocess module and wraps it so that in the end only an error message is printed, explaining which command failed.
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.
