Finally figured out the actual issue that I was trying to solve in
f38df32d and 78f06dd4. Paths like `c:/foobar` (lowercase drive
letter) are considered to be a POSIX path list, so `c:/foobar/`
becomes `c;C:\foobar`:

http://mingw.org/wiki/Posix_path_conversion

This will happen anytime we try to pass a path as a command-line
argument. We used to work-around this by using `to_unixpath()`,
but that's just sinking deeper into dependence on broken shell
behaviour. This happens now because we're passing includes in CFLAGS,
CXXFLAGS, CPPFLAGS, etc.

So let's just bypass the shell entirely by constructing a list. As
a side-effect, this is another step towards being able to run Cerbero
from a directory with spaces in it.

For now, we'll convert `config_sh` to a list when not using an
Autotools configure. Luckily, we don't need to pass any paths as
arguments to it anyway. When running other stages, we now always use
a list instead of a string. Hopefully the fallout is small and obvious
for people using forks of Cerbero.