generate-exceptions,include/xapian/: Use XAPIAN_IN_XAPIAN_H rather
than XAPIAN_INCLUDED_XAPIAN_H in the direct inclusion of subheader
checks.  Currently neither will catch the case of the user including
a sub-header after <xapian.h> since the include guard for the
subheader will stop us getting to the direct inclusion check, but
XAPIAN_IN_XAPIAN_H will work if we move the check outside the
include guard.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18228 6b2f1b92-a4f3-0310-943f-de3cbe41748b