build: do not use VPATH builds with make distcheck

make distcheck expects NEWS (and possibly other files) in srcdir while
I am autogenerating them in builddir, so it will fail with:

make[1]: Entering directory '/builds/libadg/adg/build'
sed: can't read ../NEWS: No such file or directory
NEWS not updated; not releasing

Do an in-srcdir build solves the issue.