don't depend on bsd.prog.mk & co; first step to make it portable.

the makefile now is only there to build/install the got assets

The idea is to use this cache to spot changed threads and re-export all the mail of the threads that changed. This will allow to add thread navigation hints on each exported mail and have them up-to-date. This makes the "skip if already exported" logic in mexp redundant and incomplete. pe will decide whether to export or not, and mexp blindly do the work.

this wasn't a good idea as the mbox contains sensitive data (IP addresses, hostnames and whatnot). Not good to publish on the web. Reminded by Stefan

move the logic of the export to gotmarc, where it's easier to set up the correct env. While here improve the makefile to generate the images and the copy the style only if needed.

mexp is the slowest part of the pipeline. Try to speed it up by paralellizing it. this adds a `pe' scripts that sits in front on N mexp childs and dispatch threads to them, while still outputting the unchanged lines for mkindex. Unscientific testing has shown that this effectively reduces the time for a full export, even if not linearly. (4 jobs cut the time in half, 8 jobs was just barely faster)