expose link jobs of cmake/meson

[nekopsykose]

cmake

-DCMAKE_JOB_POOLS=mycustomthing=somenumber
-DCMAKE_JOB_POOL_LINK=mycustomthing

meson

-Dbackend_max_links=somenumber

config

[build]
# link jobs in meson/cmake; defaults to jobs
link_jobs = 0

useful since by default lld thinlto will use all your threads but also link jobs are equal to thread count, so in qemu/mesa you get very high load for no reason. would be useful to be able to set it to =4 or similar locally