trigger rebuild if buildflags change

If you run `make PKG=package VIG=0 check` and subsequently
`make PKG=package VIG=1 check` the `${TARGZ}` was not rebuild and `R CMD check`
complains about a missing vignette again.

This commit adds:
  - a new variable `PKGBUILDFLAGSFILE` (default: `/tmp/${PKG}.buildflags`) that
    stores the last used buildflags (only `${VIG}` for now).
  - the target `${PKGBUILDFLAGSFILE}` that contains the logic to trigger
    the rebuilds.
  - the pseudo target `force` to always trigger reevaluation of targets (e.g.
    `${PKGBUILDFLAGSFILE}`).

Now you can run `make PKG=package VIG=0 check`, subsequently
`make PKG=package VIG=1 check` and the `${TARGZ}` would be rebuild correctly.

Obviously this results in a rebuild after cleaning `/tmp` (e.g. after reboot).
To circumvent this you can change the `${PKGBUILDFLAGSFILE}` in your
`~/.makerrc` to some non-temporary path.