Incremental builds using a sqlite3 database

Several hacks are involved:

* A fairly recent version of sqlite3 is needed (3.8.3,
  release 2014-02-03).  This version is not available
  out-of-the-box on Mac OS X, hence hacks were added to
  include the right version (for my machine).

* Database corruption will likely result in crashes or
  unexpected behaviour.  In particular, duplicate answer
  rows will cause problems.

* The database architecture is not well-documented.

* The code is not unit tested at all.

* The code is not well-factored.  (Database.c is over 1200
  lines long.)

* In the example, two files are identical if the sum of
  every byte in each file is the same (mod 2**64).  This is
  not production-worthy.