This allows cargo to use make's jobserver when building packages, by
marking the cargo command as recursive (with the + prefix[1]) and
setting MAKEFLAGS.
This also:
* Give cargo/x.py the build directory instead of having to change the
current directory (and opening subshells)
* Set PKG_BUILD_PARALLEL/HOST_BUILD_PARALLEL for Rust packages to enable
the use of make's jobserver
[1]: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html
Signed-off-by: Jeffery To <jeffery.to@gmail.com>
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