Assorted fixes:
- in test/evergreen/evg_cfg.py, allow repos cloned anonymously from github
- minor fixes for doxygen 1.8.20
- fix dist/s_export to work with both the autotools and cmake configury
- some minor portability fixes
- don't allow ancient versions of swig
- fix build failure seen with gcc8 and gcc10 and -Werror=sign-conversion
- build config for netbsd on x86
- fix test/random_directio to honor the configured O_DIRECT blocksize
- typo in build_posix.dox