compile with all available cpu cores

[divinity76]

on multi-core systems, this may significantly speed up the git compiling process.
on my laptop with 4 cpu cores rolling Intel Core i7-8565U, this speeds up the git compile process from
real 3m6,341s
to
real 1m17,921s

(that's reduction of 57% ! but varies widely by cpu)

• PS this may also significantly increase RAM consumption while compiling on multi-core systems, but generally there is a correlation between system RAM and number of cores (ie systems with lots of cpu cores usually have lots of ram as well)