move external builds into subfolder (fdo#70393)

[bjoern]

Hi,

is there any violent opposition to:

 https://bugs.freedesktop.org/show_bug.cgi?id=70393

if so, we can discuss on the ESC on Thurday. If nobody objects in 24 hours, Ill
make that an Easy Hack and push:

 https://gerrit.libreoffice.org/#/c/6231/

to master.

Best,

Bjoern