Fix make dist in a separate build directory without git

Fix make dist in a separate build directory without git

In that case, make copied the sources.mk from $srcdir, but unless it
existed already, the peculiar workings of VPATH did us in: make
searched for the target, found it in $srcdir, and the dependency
became circular.

Fix by keeping sources.mk in $srcdir always.  We can build it there,
because its contents depends only on git state, not on anything in the
build tree.  This avoids the need to copy sources.mk alltogether.