Error on `npm install -g generator-nod`

Hi thanks for this amazing repo!

Ran into the following error while trying to install the generator. I think it's related to: https://github.com/nodegit/nodegit/issues/1347

Can anybody confirm? Thanks!

- macOS 10.13.2
- node 8.7.0
- npm 5.5.1

```
➜ ~ npm install -g generator-nod

> nodegit@0.20.3 install /usr/local/lib/node_modules/generator-nod/node_modules/nodegit
> node lifecycleScripts/preinstall && node lifecycleScripts/install

[nodegit] Running pre-install script
[nodegit] Configuring libssh2.
{ Error: Command failed: /usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/configure --with-libssl-prefix=/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/openssl/openssl
/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing: Unknown `--is-lightweight' option
Try `/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing --help' for more information
configure: WARNING: 'missing' script is too old or missing
configure: error: No crypto library found!
Try --with-libssl-prefix=PATH
or --with-libgcrypt-prefix=PATH
or --with-wincng on Windows

at ChildProcess.exithandler (child_process.js:271:12)
at emitTwo (events.js:125:13)
at ChildProcess.emit (events.js:213:7)
at maybeClose (internal/child_process.js:927:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:211:5)
killed: false,
code: 1,
signal: null,
cmd: '/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/configure --with-libssl-prefix=/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/openssl/openssl' }
/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing: Unknown `--is-lightweight' option
Try `/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing --help' for more information
configure: WARNING: 'missing' script is too old or missing
configure: error: No crypto library found!
Try --with-libssl-prefix=PATH
or --with-libgcrypt-prefix=PATH
or --with-wincng on Windows

[nodegit] ERROR - Could not finish preinstall
{ Error: Command failed: /usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/configure --with-libssl-prefix=/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/openssl/openssl
/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing: Unknown `--is-lightweight' option
Try `/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/missing --help' for more information
configure: WARNING: 'missing' script is too old or missing
configure: error: No crypto library found!
Try --with-libssl-prefix=PATH
or --with-libgcrypt-prefix=PATH
or --with-wincng on Windows

at ChildProcess.exithandler (child_process.js:271:12)
at emitTwo (events.js:125:13)
at ChildProcess.emit (events.js:213:7)
at maybeClose (internal/child_process.js:927:16)
at Process.ChildProcess._handle.onexit (internal/child_process.js:211:5)
killed: false,
code: 1,
signal: null,
cmd: '/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/libssh2/configure --with-libssl-prefix=/usr/local/lib/node_modules/generator-nod/node_modules/nodegit/vendor/openssl/openssl' }
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! nodegit@0.20.3 install: `node lifecycleScripts/preinstall && node lifecycleScripts/install`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the nodegit@0.20.3 install script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! ~/.npm/_logs/2017-12-25T09_46_12_726Z-debug.log
```

Start by reproducing `npm install -g generator-nod` with the reported macOS, Node, and npm versions, then inspect the nodegit@0.20.3 install step and the referenced nodegit issue. The error centers on the bundled libssh2 configure script and missing crypto library; done requires confirming the cause and identifying an actionable project change or documented compatibility limitation.

Written by the indexing model from the issue text.