build: ./configure pass positional args to gyp

use `--` to specify the arguments you want to pass directly to gyp.

for example: `./configure -- --no-parallel -Dsome_define=foo`

fixes #6370
build: pass --no-parallel by default to gyp

gyp by default now tries to process gyp files in parallel by using
python's multiprocessing module, but it has problems on oddball
platforms. We don't have many files or complex dependency chains that
would benefit from parallel processing so disable by deafult

fixes #6640