xapian-head.i,xapian-headers.i: Define XAPIAN_IN_XAPIAN_H rather
than XAPIAN_LIB_BUILD to avoid the direct subheader inclusion
checks.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18229 6b2f1b92-a4f3-0310-943f-de3cbe41748b