adding @DataDrake for the okay to use it in the way

My results with LLVM_PARALLEL_LINK_JOBS=1 on an FX-8350 w/16 GiB RAM + 4GiB swap:

-j2:

real    206m23.818s
user    383m30.154s                                                                                                                                 
sys     6m24.131s

-j3: (max RAM usage by compile jobs was 2GiB + 6-ish GiB + 7.5-ish GiB)

real    162m49.967s
user    412m23.566s
sys     6m40.476s

So decent scaling with -j2 (~86%) and -j3 (~85%). I wouldn't go higher than -j3 on a 16 GiB system if I was planning on working at the same time. Crazy to think that -j1 on Girt's box is quicker than -j2 on mine. =)

-j4 is probably more than fine with LLVM_PARALLEL_LINK_JOBS=1 on 32GiB systems (worked fine on Girt's 64 GiB box).

The highest link job memory usage I saw was 11.5-ish GiB, but since that's just one at a time now due to make's depth-first approach, that's a non-issue.

With what we now know, I suppose this knowledge could be captured in the following comment in the build section:

# Maximum observed single compile job size is around 8 GiB Resident with sizes in the range 1.5-5 GiB the most common.
# Likely due to poor job parallelism in make, the build will fan out to $CPUs -> 4-6 CPUs -> 1-2 CPUs.
# For systems with 16 GiB RAM, make -j3 is barely suitable and make -j2 is recommended if the system will be in use during the build.
# For systems with 32 GiB RAM, make -j8 is probably the maximum.
# For systems with 64 GiB RAM, make -j24 is probably the maximum, but won't be appreciably faster than make -j8
make %JOBS%