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.
This fixes exporting SVG files which contain text on a path where the path is on a different layer than the text (which is necessary unless the path should also be combined with the text in a single layer).
