Build does not respect make -jN

[hlein]

Building with make -jN ought to pass that N down to children, but instead we use /usr/bin/nproc --all.

From https://bugs.gentoo.org/924381 :

ps -faux
...
portage  25699  0.0  0.0  10684  4096 ?        SN+  22:26   0:00    |               \_ make -j4 static
portage  10053  0.0  0.0  15248  8696 ?        SN+  22:26   0:00    |                   \_ make -j 32 -C openssl build_libs

I think we should respect if we're already running under a jobserver, and not re-calculate a -j arg in that case.