configure: Set XAPIAN_CONFIG to only "xapian-config" by default.

configure: Set XAPIAN_CONFIG to only "xapian-config" by default.

Previously, we preferred a value of "xapian-config-1.1" first. This
was convenient for compiling against Xapian 1.1 while Xapian 1.2 was
unreleased. But now that Xapian 1.2 is realease, and since it ships a
xapian-config, the xapian-config-1.1 value can mask the newer library.

Instead of trying to track the latest xapian-config-1.x in our
configure script let's simply expect the user to set
XAPIAN_CONFIG=xapian-config-1.x in order to compile against an
unreleased Xapian.