Multiple ORACLE installations - "make" errors

If you are running multiple installations on one server and you get make errors similar to below:

Unable to find make file: /oracle//102_64/network/lib/ins_net_client.mk
etc
SEVERE:OUI-10105:Some of the configuration assistants failed. However these are optional assistants, so they are not required for the correct configurati
on of your system. If you want to try to run those assistants again, select the failed assistants and click the 'Retry' button.

Then the fix is simple. You may, like I was, be using a single oraInventory location, which inventories details of installations. However if you manually uninstall or muck it around at all it's going to get very unhappy.
The the very least I suggest editing it for each installation to have a separate directory.
For example instead of just using /oracle/oraInventory for six db SIDs, use
/oracle/oraInventory/SID1dir
/oracle/oraInventory/SID2dir
/oracle/oraInventory/SID3dir
/oracle/oraInventory/SID4dir

Note the inventory location is set in /var/opt/oracle/oraInst.loc

For SAP ORACLE installations see SAP note: 350251