Everything in src/lib/ is under MIT license (see COPYING.MIT) unless
otherwise mentioned at the beginning of the file.

Everything else is LGPLv2.1 (see COPYING.LGPL) unless otherwise mentioned
at the beginning of the file.

Current exceptions are:

src/lib/
  - md5.c : Public Domain
  - base64.c, utc-mktime.c : BSD-like (read it)
  - primes.c, printf-upper-bound.c : LGPL v2

src/imap/imap-thread.c : merge sorting code is with MIT license.