remove INPATH and PROEXT

- WORKDIR path is just workdir
- INSTDIR path is just instdir
- WORKDIR_FOR_BUILD is workdir_for_build
- INSTDIR_FOR_BUILD is instdir_for_build
- replace other usage of INPATH by combination of OS and CPUNAME

Change-Id: Ie398387ebd82a968ec2605f2103c55b43a231482
Reviewed-on: https://gerrit.libreoffice.org/6601
Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>
Reviewed-by: Michael Stahl <mstahl@redhat.com>