Relocate libtool (.la) files as well as pkgconfig (.pc)

I've been thinking about this whole regexp stuff, especially in light of my desire to do this work to `.la` files (such as those in `gdk-pixbuf`) that have lines such as the following:

```
dependency_libs=' "/usr/local/Cellar/gdk-pixbuf/2.30.1/lib/libgdk_pixbuf-2.0.la" -L"/usr/local/Cellar/glib/2.38.1/lib"
-L"/usr/local/opt/gettext/lib" -L"/usr/local/Cellar/libpng/1.5.14/lib" -lgmodule-2.0 -lgio-2.0 -lgobject-2.0 -lglib-2.0
-lintl -lpng15 -lm'
```

Clearly this won't be caught by [our current fancy regexp](https://github.com/mxcl/homebrew/blob/8cf402801621b5fee9b0852d68348a5e277a9de9/Library/Homebrew/keg_fix_install_names.rb#L67).  This pull request changes up the regexp to not include the `variable_name=` prefix.  It also adds searching through `.la` files, and fixes a problem with quoting where single quotes weren't being caught.

Thoughts?