Make number of Rerun loops to generate targets lex directory more flexible

Till now it was only possible to change the Makefile or the make.bat in case the default number of Rerun-loops was insufficient. With this patch the number of loops can also be specified from outside (e.g. make 8 when using make.bat or make latex_count=10  when using Makefile. Advantage is that this value can easier be applied in scripts / other Makefiles.