automatically find the OpenSCAD binary on Mac OS X
renable DRAWEXE exporter

This reverts commit 4c48bdf2166f273e60bcaf95281b7dde726db2f5.
support for Part::Loft in exportDRAWEXE
remove unused meshing paramters for exportCSG

releated to 382e055e9b2d3159de79e3f56ef452b538b51012