[PATCH 1/4] Use the Xapian::DB_RETRY_LOCK flag when available

This is the original patch, rebased on master. I took the liberty of
converting the configure test to a test compile.

[PATCH 2/4] test: factor out some boilerplate from C tests
[PATCH 3/4] test: initial tests for locking retry

Add a simple test of locking retry.

[PATCH 4/4] lib: add built_with handling for XAPIAN_DB_RETRY_LOCK

Make it visible to the user whether their notmuch supports locking
retry (blocking open).

The remaining question for me is if we are happy with defaulting to
blocking (w/o timeout) on open. I'm not sure how you could deadlock
without writing C code, but it's pretty easy with fork. As I
discovered writing my test case.