[PATCH] build: drop support for xapian versions less than 1.4

[David Bremner]

Tomi Ollila <tomi.ollila at iki.fi> writes:

> Xapian 1.4 is over 3 years old now (1.4.0 released 2016-06-24),
> and 1.2 has been deprecated in Notmuch version 0.27 (2018-06-13).
>
> Xapian 1.4 supports compaction, field processors and retry locking;
> conditionals checking compaction and field processors were removed
> but user may want to disable retry locking at configure time so it
> is kept.

Hi Tomi;

I didn't understand why yet, but I'm consistently getting a test failure
with your patch.

T670-duplicate-mid: Testing duplicate message ids
 BROKEN First subject preserved in notmuch-show (json)
 FAIL   Regexp search for second subject
	--- T670-duplicate-mid.4.EXPECTED	2020-04-02 10:10:39.461044592 +0000
	+++ T670-duplicate-mid.4.OUTPUT	2020-04-02 10:10:39.461044592 +0000
	@@ -1,3 +0,0 @@
	-MAIL_DIR/copy0
	-MAIL_DIR/copy1
	-MAIL_DIR/copy2

Does the test pass for you?

David