[PATCH v5 00/15] build improvements

[Johannes Berg]

Many thanks to Felix :-)

Changes since v4:
 - put archives into build/ folder too, as they can differ (config)
 - allow overriding BUILDDIR on the command line
 - fix some bugs, particularly avoid the use of $(PWD)
 - make it all work on ancient make (3.81beta4, 15 years old)
 - add more .gitignore, since I was building _everything_ I could
 - use the new system for tests and fuzz tests
   (except tests/*-fuzzer which mostly don't build for me even on master)

johannes