The build guide doesn't mention it. https://wiki.openoffice.org/wiki/Documentation/Building_Guide_AOO . I tried to add the following information, but got a 502 error:
|- valign="top" | unowinreg.dll | [http://tools.openoffice.org/unowinreg_prebuild/680/unowinreg.dllpre-built unowinreg.dll] (see note) note: Not for windows. Required on all non-windows platforms. Download with: cd $SRC/main && wget -O external/unowinreg/unowinreg.dll http://tools.openoffice.org/unowinreg_prebuild/680/unowinreg.dll -- Marcin Tustin
