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.