Changes to fetchDependencies script:
- remove -sdk option
- Add --v-lvl-root and --glslang-root options (with an arg)
that allow caller to pass in the root dir of these two repos
if the caller already has them. The script then symlinks to
these instead of fetching and building.
Fixes: #79#84
fetchDependencies script now in top directory to avoid Travis caching of
External directory. fetchDependencies is now smart about fetching vs cloning
and building only what is necessary.