build: do not use VPATH builds with `make distcheck`
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.

ntd/adg,ntd/adg