currently not able to install at tip of master - OS X 10.12.5

At the tip of the master branch (8d5479a), I found that `build.ninja` had a reference to windows paths for `bsc` and `refmt`. By chance is master waiting for an additional piece such as your PR for `reasongl-ios` to [`matchenv`](https://github.com/bsansouci/matchenv/pull/1/commits/06f3586766d90d7874b2e9e1f9540f915a1ae4db)?

```
$ yarn install
yarn install v1.3.2
warning package.json: License should be a valid SPDX license expression
warning gravitron@0.1.0: License should be a valid SPDX license expression
[1/4] 🔍 Resolving packages...
[2/4] 🚚 Fetching packages...
warning Pattern ["bs-platform@bsansouci/bsb-native#2.1.1"] is trying to unpack in the same destination "/Users/pnore/Library/Caches/Yarn/v1/npm-bs-platform-2.1.1-d76155b709dba032cdb11e99ebe0e2c37bcb2c85" as pattern ["bs-platform@github:bsansouci/bsb-native#2.1.1"]. This could result in a non deterministic behavior, skipping.
warning Pattern ["bs-platform@bsansouci/bsb-native#fast"] is trying to unpack in the same destination "/Users/pnore/Library/Caches/Yarn/v1/npm-bs-platform-2.1.1-d76155b709dba032cdb11e99ebe0e2c37bcb2c85" as pattern ["bs-platform@github:bsansouci/bsb-native#2.1.1"]. This could result in a non deterministic behavior, skipping.
warning Pattern ["bs-platform@github:bsansouci/bsb-native#fast"] is trying to unpack in the same destination "/Users/pnore/Library/Caches/Yarn/v1/npm-bs-platform-2.1.1-d76155b709dba032cdb11e99ebe0e2c37bcb2c85" as pattern ["bs-platform@github:bsansouci/bsb-native#2.1.1"]. This could result in a non deterministic behavior, skipping.
[3/4] 🔗 Linking dependencies...
[4/4] 📃 Building fresh packages...
[1/7] ⢀ sdl2
[2/7] ⠠ ppx-env: ninja: error: rebuilding 'build.ninja': subcommand failed
[3/7] ⠠ matchenv: ninja: error: rebuilding 'build.ninja': subcommand failed
[4/7] ⠠ bs-platform
error /Users/pnore/dev/react/gravitron/node_modules/matchenv: Command failed.
Exit code: 2
Command: bsb && cp lib/bs/native/index.native matchenv
Arguments:
Directory: /Users/pnore/dev/react/gravitron/node_modules/matchenv
Output:
ninja: Entering directory `lib/bs'
[1/3] Building Matchenv.cmi
[2/3] Building src/index.mlast
FAILED: src/index.mlast
/usr/local/lib/node_modules/bs-platform/bin/bsc.exe -pp "/usr/local/lib/node_modules/bs-platform/bin/refmt.exe --print binary" -w -40+6+7+27+32..39+44+45 -nostdlib -I '/Users/pnore/dev/react/gravitron/node_modules/bs-platform/lib/ocaml' -no-alias-deps -color always -c -o src/index.mlast -bs-syntax-only -bs-binary-ast -impl /Users/pnore/dev/react/gravitron/node_modules/matchenv/src/index.re
File "/Users/pnore/dev/react/gravitron/node_modules/matchenv/src/index.re", line 11, characters 17-19:
Error: 1742: Incomplete statement. Did you forget a ";"?

File "/Users/pnore/dev/react/gravitron/node_modules/matchenv/src/index.re", line 1:
Error: Error while running external preprocessor
Command line: /usr/local/lib/node_modules/bs-platform/bin/refmt.exe --print binary '/Users/pnore/dev/react/gravitron/node_modules/matchenv/src/index.re' > /var/folders/vd/_cgkbpk16sx746c9_0cf3r

```

Start by reproducing `yarn install` on OS X 10.12.5 at commit `8d5479a`, focusing on the generated `build.ninja` and the `matchenv` package build. Compare the `bsc` and `refmt` paths in the failing command; done means installation completes without Windows paths or the reported `index.re` preprocessing failure.

Written by the indexing model from the issue text.