Currently, a lot of our build variables that are defined at
configuration time, are spread out across the code base, often declared
as `extern` which can break when moving around sections of the code
across files.
This commit introduces a "Config" namespace which basically maps to the
definitions in `config.h`, but allows us to properly access them too
from the Vala source code.
By doing so, it helps us to more explicitly see where we rely on a
build variable from this file (which should be obvious from the `Config'
namespace).
To make it ourselves a bit easier in Meson too, we can declare an
internal dependency, which helps ensure that we pull in the dependency
where needed.
Revert merge request GNOME/geary!374 for now since the shared process
model breaks old-style WebProcess message handler IPC.
This can be un-reverted when out JS is ported to the new Messages API
that is landing in WebKitGTK 2.28.
This reverts commit e4a5b85698, reversing
changes made to 66f6525480.
