message: Clarify behavior of DESTDIR in the Makefile
diff: ```
  * `make install` respects `DESTDIR`, but `prefix` must be configured
  - with `WASMER_INSTALL_PREFIX`, e.g.:
  + with `WASMER_INSTALL_PREFIX`. Note that `DESTDIR` must include
  + `WASMER_INSTALL_PREFIX`, e.g.:
  ```sh
  export WASMER_INSTALL_PREFIX=/usr
  make
  - DESTDIR=.../usr make install
  + DESTDIR=/tmp/staging/usr make install
  ```
  * In case you must build/install directly with `cargo`, make sure to
  