Officially support more typical CMake build folder position

[ssokolow]

mkdir ../Retro68-build needlessly clutters up my ~/src/read-only folder by breaking the "one folder, one project" rule unless I add an extra layer of nesting to paths that can already get awkwardly long, so I decided to go for this more typical-for-CMake-projects layout instead:

cd /path/to/checkout/of/Retro68
mkdir build
cd build
../build-toolchain.bash

It seems to work fine for me. Could it be an officially supported option and noted as such in the README?

[autc04]

Yes, there is no reason any more why one shouldn't use a standard layout.