Add instructions on dealing with MSB4019

I saw this error when trying to install a project that depended on utf-8-validate:

```
{my-node-project-dir}\node_modules\bufferutil\build\bufferutil.vcxproj(20,3): error MSB4019: The imported project "C:\Program Files(x86)\MSBuild\Microsoft.Cpp\v4.0\V110\Microsoft.Cpp.Default.props" was not found. Confirm that the path in the <Import> declaration is correct, and that the file exists on disk.
```

Despite having followed all the instructions listed in this README, the `C:\Program Files(x86)\MSBuild\Microsoft.Cpp\v4.0\V110\Microsoft.Cpp.Default.props` directory didn't exist (it should've been V140, which corresponds to VS 2015).

The solution to this problem was at the bottom of [this issue filed against node-gyp](nodejs/node-gyp#671), and I thought it'd be useful if it was also highlighted here.

Author: ivantm

windows-environment.md

@@ -102,7 +102,9 @@ Here are a few packages you can try installing to see if your environment is set
 | **Inability to find msbuild, Visual Studio, or VC compiler** | VC compiler not installed, or environment not properly configured | <ul> <li>Install VC++ compiler</li> <li>Specify --msvs_version=2015 (or other VS version)</li> <li>npm config set msvs_version 2015 -g</li> </ul>                                                                                                                                                                                                                                                       |
 | **NaN/Node/v8/iojs-related syntax errors**                   | Package incompatible with current version of Node.js              | <ul> <li>Upgrade to latest version of package + node.js, and see if issue still exists</li> <li>Search issues and/or file an issue on package repo</li> </ul>                                                                                                                                                                                                                                           |
 | **Other syntax errors**                                      | Incompatible with compiler version                                | <ul> <li>Upgrade to latest version of package + node.js, and see if issue still exists</li> <li>Search issues and/or file an issue on package repo</li> </ul>                                                                                                                                                                                                                                           |
-| **Missing command or *.h file**                              | Configuration is probably fine, but missing other prerequisites   | <ul> <li>Upgrade to latest version of package</li> <li>Check docs, try to install missing prerequisites, ensure they're accessible in PATH</li> <li>Search for header file or other pre-requisite that's missing, that may provide a clue where it's supposed to come from (e.g. Windows SDK not installed, OpenSSL, etc.)</li> <li>Search issues and/or file an issue on package repository</li> </ul> |
+| **Missing command or *.h file**                              | Configuration is probably fine, but missing other prerequisites   | <ul> <li>Upgrade to latest version of package</li> <li>Check docs, try to install missing prerequisites, ensure they're accessible in PATH</li> <li>Search for header file or other pre-requisite that's missing, that may provide a clue where it's supposed to come from (e.g. Windows SDK not installed, OpenSSL, etc.)</li> <li>Search issues and/or file an issue on package repository</li> </ul>
+                                  |
+| **MSB4019 error**                                      | Older versions of Visual Studio or C++ Build tools already installed                               | <ul> <li>Add or modify the environment variable `VCTargetsPath` top point at the C++ build tools path. This should be something like `C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\v140` (where 140 corresponds to Visual Studio 2015)</li> <li>Search issues and/or file an issue on package repo</li> </ul>                                                                                                                                                                                                                                           |
 
 #### Deploying native modules
 Sometimes, when deploying a native module to production, oftentimes it is not possible to set up the production machine with all the required prerequisites to build the native Addon. Therefore, building locally or on a CI server and deploying `node_modules` may be the best option assuming there aren't any platform differences between the development and deployment machines.