Avoid using sed -i in makedoc.g

[fingolfin]

... as it is not portable between GNU sed and BSD sed (as used on e.g. macOS).
Possible alternatives:

• avoid it altogether: in the first usecase it is used to remove leading whitespace form a file we just generated. Instead of that, simplify modify the string cont suitably before printing it out (e.g. search for the substrings of the form \n or \n\t or \n\t and replace them by \n: Say via cont := JoinStringsWithSeparator(List(SplitString(cont,"\n"), line -> StripBeginEnd(line, " \t")), "\n"); -- this would also remove trailing whitespace, which I think is desirable here
• use perl -pi -e EXPR (but then you need perl, which is a much heavier dependency than sed)
• instead of using -i to enable in-place editing, just pipe the output of sed into a separate file