[RFC] Fix installation.

Installation is broken with #1379/#1381: `make install` adds an additional directory (in this case $HOME; see output below).

```
$ make DESTDIR=$HOME/nvim
...
$ ll $HOME/nvim
total 0
drwxr-xr-x 1 florian users 14 Nov  3 18:11 home
drwxr-xr-x 1 florian users 10 Nov  3 18:11 usr
$ ll $HOME/nvim$HOME/nvim/usr/local
total 0
drwxr-xr-x 1 florian users  0 Nov  3 18:11 bin
drwxr-xr-x 1 florian users 20 Nov  3 18:11 share
```

I'd thus suggest reverting this first to fix `master` and finding a solution for https://github.com/neovim/neovim/pull/1379#issuecomment-61444684 afterwards, maybe @jszakmeister can help. Ping @justinmk.
If CMAKE_INSTALL_PREFIX is an absolute path (the default use case, I'd say; there were never any bugs reported because of this [#1368 was about fortify]), it doesn't matter, at least on *nix systems.

I tried to just send a quick fix, because at the moment all installs are broken. I discovered it because I tried to update Neovim from the Arch Linux package and it wanted to install files into `/tmp`.

I had a bit more time to look at it; updated the PR.