Assorted Fixes

This is a bunch of small fixes, mostly to the build system.

* Add vim temporary files to .gitignore (`.*.s??`)
* Adds a LINK_PAM option to force `libpam` to be linked. The detection isn't 100% and fails on my machine, however, there does not seem to be a way to detect another way
* ~~Removes the DEEPBIND flag in the call to `dlopen` for loading extensions. This seems to be the cause of #1174. At the very least, its removal fixed the issue for me~~
* ~~Stops CMake from using rpath. This was causing it to relink `hhvm` before installation, resulting in the systemlib disappearing, breaking `make install`.~~
* Adds a link dependency to the target of an `embed_systemlib` call. It has to be a link dependency, as the custom command is run as a `POST_BUILD` command, meaning it cannot have dependencies added. This should fix #1180.