HACKING: Update details of building Xapian packages.

(cherry picked from commit a452249e20d3ef2932f7e945190a9fbb1210f984)
configure.ac: Update link to cygwin FAQ.

(cherry picked from commit cbf77ed9392e65221aa54cbc7aa64f165c178c1d)
docs/doxygen_api.conf.in,docs/doxygen_source.conf.in: Update to
avoid warnings about obsolete tags from newer doxygen.

(cherry picked from commit e1b231a9ac2e8c96c29c1f231408a24474a8bcd9)