 cannot open file: compiler/ast
When nim compiler is installed systemwide (whether it's from a debian package on Ubuntu, or from an ebuild on Gentoo) attempting to `nimble install nimterop` only gets me this:

```
     Error: Build failed for package: nimterop
        ... Details:
        ... Execution failed with exit code 1
        ... Command: "/usr/bin/nim" c --noNimblePath -d:NimblePkgVersion=0.6.7 -d:release --path:"/home/shirow/.nimble/pkgs/regex-0.15.0"  --path:"/home/shirow/.nimble/pkgs/unicodedb-0.9.0"  --path:"/home/shirow/.nimble/pkgs/unicodeplus-0.8.0"  --path:"/home/shirow/.nimble/pkgs/unicodedb-0.9.0"  --path:"/home/shirow/.nimble/pkgs/segmentation-0.1.0"  --path:"/home/shirow/.nimble/pkgs/unicodedb-0.9.0"  --path:"/home/shirow/.nimble/pkgs/cligen-1.1.0"  -o:"/tmp/nimble_9544/githubcom_genotrancenimterop_#head/nimterop/toast" "/tmp/nimble_9544/githubcom_genotrancenimterop_#head/nimterop/toast.nim"
        ... Output: Hint: used config file '/etc/nim/nim.cfg' [Conf]
        ... Hint: system [Processing]
.....skipping a long list of hints for brevity.......
        ... Hint: plugin [Processing]
        ... /tmp/nimble_9544/githubcom_genotrancenimterop_#head/nimterop/globals.nim(8, 18) Error: cannot open file: compiler/ast
```

Even to get this error message, first I must agree to nimble installing `#head`, because the tagged version fails first. *(This happened with both 0.6.6 and 0.6.7)*

Ironically, checking out nimterop and building `toast` directly with `nim c nimterop/toast` on Gentoo results in no problems at all:

```
CC: toast.nim                                 
Hint:  [Link]           
Hint: 191083 LOC; 85.134 sec; 271.414MiB peakmem; Debug build; proj: nimterop/toast; out: /home/shirow/nimterop/nimterop/toast [SuccessX]  
```

However, `nimble develop -y`, `nimble build -d:danger` still doesn't work:

```
        ... Hint: sequtils [Processing]
        ... Hint: plugin [Processing]
        ... /home/shirow/nimterop/nimterop/globals.nim(8, 18) Error: cannot open file: compiler/ast
```

When I install nim into the depths of the home directory with `choosenim`, I have no issues, and `nimble install nimterop` completes. However, this is not always an desirable option -- for example, writing an ebuild to compile a nimterop-using package on Gentoo would become really difficult if that was the only way to get it to work.

You would think `nimble install compiler` would help, but it doesn't, until I manually add it to requirements in `nimterop.nimble`

As far as I could determine, `choosenim` drags the `compiler` library in with the rest of the standard library and places it where the compiler can import it.

* On Gentoo, the systemwide nim installation puts it into `/opt/nimble/pkgs/` instead, which makes at least `nimble` lose it even when `nim c` has no issues.
* On Ubuntu, the deb does not appear to contain `compiler` *at all.* And since everything else about nim goes into `/usr/lib/nim`, I'm not sure where `compiler` is *supposed to* go in the first place.

I'm not sure who is in the wrong here, the packagers or nimterop. But I imagine it should be possible to detect this situation in `nimterop.nimble` somewhere and simply require `compiler` at an appropriate version if it is not otherwise available?