Allow dropin buildflags for other programming languages
Splits out rust support as a demo of how distros may want to add support for setting build flags for other programming languages.
Two components:
- makepkg.conf.d/rust.conf - adds support for drop-ins for makepkg.conf
- libmakepkg/buildenv/rust.sh - demo of libmakepkg drop-in to handle !buildflags etc