$(PKGBASE) doesn't resolve because it's not defined in the code.
It doesn't help us anyway, the linux header packages in SteamOS are 'non standard', you would expect them all to be included in 'linux-headers'?
In fact, they are not. They are stored in packages named similar but not exactly to the steamOS version name. Instead we should tell people in the error/warning message to search for the version themselves and make an educated guess based on our output of where we expected the headers to be found.
For example at the time of writing, the correct header package would be:
$(PKGBASE) doesn't resolve because it's not defined in the code. It doesn't help us anyway, the linux header packages in SteamOS are 'non standard', you would expect them all to be included in 'linux-headers'? In fact, they are not. They are stored in packages named similar but not exactly to the steamOS version name. Instead we should tell people in the error/warning message to search for the version themselves and make an educated guess based on our output of where we expected the headers to be found.
For example at the time of writing, the correct header package would be:
jupiter-3.5/linux-neptune-61-headers