Allow building against Glib 1.x.

In reality, we barely depend on any glib-2 features in the tool
itself (of course, the output does require it).

Other than very minor additions and pointlessly renamed functions,
the only real new feature is the locale-independent g_ascii_xxx
case conversions.  As we already have an abstraction of these in
place it is very easy to just substitute a gnulib-based replacement
when configure determines they are unavailable.

At the moment, configure does not know how to automatically find
glib-1.x, so manually setting LIBGLIB_CFLAGS and LIBGLIB_LIBS is
required to build against it.  This is probably fine.