[RFC] Require SQLite >= 3.6.18

Any objection to requiring SQLite >= 3.6.18 as in the attached patch?

WHY?

In order to use SQLite triggers to automatically update the pristine
text reference counts (see the thread "Ref-counting for pristine texts":
<http://svn.haxx.se/dev/archive-2011-01/0104.shtml>), we have to either

  * enable recursive triggers, which requires SQLite >= 3.6.18

or

  * split each of our current "INSERT OR REPLACE" statements into a
DELETE statement followed by an INSERT statement.

The former solution is simpler, which is enough of an advantage for me.
I haven't yet found other significant pros or cons.

AVAILABILITY

Subversion can easily be compiled with a sepcific version of SQLite, by
including the SQLite source amalgamation file in the Subversion source
directory.

Some Linux distros include a recent enough version of SQLite already.
Here are some Linux distros with the versions of SQLite they ship. (I
found these on the distributions' web sites; I couldn't find version
information for RedHat/Fedora.)

Suse SLED/SLES-10 - 3.2.8
Suse SLED/SLES-11 - 3.6.4
Debian lenny (stable) - 3.5.9
Debian lenny-backports - 3.7.3
Debian squeeze (testing) - 3.7.3
Ubuntu dapper - 3.2.8
Ubuntu hardy - 3.4.2
Ubuntu karmic - 3.6.16
Ubuntu lucid - 3.6.22
Ubuntu maverick - 3.7.2

- Julian