Unconditionally build GModule

We currently allow building GLib without a working GModule implementation, but we do kind of assume that GModule is available—for instance, GIO's modules are predicated on it working.

The chance of an OS existing that is both supported by GLib and does not have a GModule implementation in 2021 is vanishingly small.

My proposal is to turn:

# Done, have we got an implementation?
if g_module_impl == ''
  g_module_impl = '0'
  message('WARNING: No suitable GModule implementation found!')
endif

into:

if g_module_impl == ''
  error('No suitable GModule implementation found')
endif

and require dlopen() to exist on any POSIX-like systems. If there are operating systems with other mechanisms for dynamic module loading, then they should contribute the code to GLib.