Previously, meson test checked the MESON_TESTTHREADS variable to control the amount of parallel jobs to run; this was useful when meson test is invoked through ninja test for example. With this version, a new variable MESON_NUM_PROCESSES is supported with a broader scope: in addition to meson test, it is also used by the external_project module and by Ninja targets that invoke clang-tidy, clang-format and clippy.