deprecation of in-tree builds

[Peter Maydell]

AIUI from Paolo, the intention is to deprecate and eventually
stop supporting "in-tree" builds, so that the only option is
building in a separate build directory. I thought we should
probably mention that in the 5.0 changelog, so I wrote up some
text:

https://wiki.qemu.org/ChangeLog/5.0#Build_Information

Suggestions for changes/comments etc welcome.

(One thing we will need to fix before we can do separate build
tree is the Coverity Scan build process, which (a) does an
in-tree build (b) can't be easily switched to a builddir because
all the source paths get baked into the scan results and moving
to a builddir changes them all...)

We could also make configure actively warn if used in
the source tree.

thanks
-- PMM