Backport change from trunk:
Fix patch dependencies when srcdir != builddir

In this case, we now set "VPATH = $srcdir" in the generated Makefile.


git-svn-id: file:///home/xapian-svn/xapian/branches/1.2@18301 6b2f1b92-a4f3-0310-943f-de3cbe41748b