xfce4-dev-tools: Allow out of tree builds

The xdt-autogen target in the generated scripts/Makefile refers to
a $(srcdir)/xdt-autogen.in file.  For an out-of-tree build, this file
is actually located in $(builddir)/xdt-autogen.in. See [1]

Upstream-Status: Submitted

[1] https://bugzilla.xfce.org/show_bug.cgi?id=10899



Signed-off-by: Ash Charles <ash@gumstix.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>