Follow the same pattern as used in the cmake setup hook to assume
`enableParallelBuilding=1` when unset by the user. This makes wafHook
honor `NIX_BUILD_CORES`. See #136368 and #136641.
The convention of `--cross-compile` and `--cross-execute` is common
enough that it seems like a reasonable default. However there are
projects like mpv which do not use these flags, and rightfully fails
to configure when passed unexpected flags.