Configurable per-package `--mflags`

[rsekman]

Have you checked the readme and man page for this feature? Yes
Have you checked previous issues for this feature? Yes

It would be convenient if one could, in paru.conf, specify per-package values of --mflags. Reasons for this include

• Some AUR packages, e.g. dropbox fail to build unless the build directory is clean.
• Some AUR packages leave very large build directories. For example python-jaxlib can use upwards of 10 GB since its bazel-based build involves compiling all of LLVM.
  In these cases I would like to pass one or both of --clean --cleanbuild to makepkg to ensure successful system updates without manual intervention and/or keep the package cache from exploding in size. But for VCS packages that do work well with incremental builds, I would like to not do that. So the goal here is that one could put in paru.conf something like

[python-jaxlib]
MFlags = --clean

and paru would pass --clean when building python-jaxlib, but not for other AUR packages.

It would also be neat if --cleanafter could be configured per-package like this. Packages like dropbox and zoom download a new, large archive file for each version growing the cache for each update.