I took a quick peek through openvdb/CMakeLists.txt but it seems that only the core and the binaries are build by default.
The binaries are disabled here.

Which leaves the core, which builds only two libraries, a static and a shared. But since -j1 is already present, these should build sequential.

So not sure how to reduce the memory footprint for this compilation…

(We might gain a factor 0.5 in time by disabling the static libraries though, depending on how good ccache does it’s work).