Fix remaining warnings

[JulianGro]

This PR fixes the rest of the warnings that come from this repo. Some warnings are still coming from files that come from the main vircadia repo.

Fixes #110

A preview is available here: https://data.moto9000.moe/sphinx/PR124/ (c1e1eee)

[JulianGro]

With the last commit I added functionality to the Makefile that fixes the hyperlinks that break when importing the build documentation from the main repository.

[ctrlaltdavid]

On Windows...

• Before (i.e., current master): 10 build warnings
• After (i.e., this PR): 4 build warnings

  D:\Workspaces\CtrlAltStudio\Vircadia-Docs-Sphinx\Vircadia-Docs-Sphinx\docs\source\developer\build\BUILD.md:35: WARNING: None:any reference target not found: cmake/externals
  D:\Workspaces\CtrlAltStudio\Vircadia-Docs-Sphinx\Vircadia-Docs-Sphinx\docs\source\developer\build\BUILD.md:150: WARNING: None:any reference target not found: cmake/modules/
  D:\Workspaces\CtrlAltStudio\Vircadia-Docs-Sphinx\Vircadia-Docs-Sphinx\docs\source\developer\build\BUILD.md:178: WARNING: None:any reference target not found: interface/external
  D:\Workspaces\CtrlAltStudio\Vircadia-Docs-Sphinx\Vircadia-Docs-Sphinx\docs\source\developer\installer\INSTALLER.md:5: WARNING: None:any reference target not found: BUILD

[JulianGro]

Those warnings are fixed in the Makefile. Windows uses a batch file which I didn't even bother looking at. It doesn't seem nearly as important and Windows shell is just extremely limited. We could have people install GNU sed via chocolatey and just use the same command as on Linux, but at that point we might as well just have them install GNU make as well which probably makes Windows users able to use the Makefile.

Whatever we do, I would like a Windows person to make a PR if they really want to go through the trouble for 4 warnings.

[daleglass]

Probably should use PowerShell instead for Windows builds, but that's definitely for later