meson: remove config_targetos

config_targetos is now empty and can be removed; its use in sourcesets
that do not involve target-specific files can be replaced with an empty
dictionary.

In fact, at this point *all* sourcesets that do not involve
target-specific files are just glorified mutable arrays.  Enforce that
they never test for symbols in "when:" by computing the set of files
without "strict: false".

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

storage-daemon/meson.build

@@ -5,7 +5,7 @@ qsd_ss.add(blockdev, chardev, qmp, qom, qemuutil, gnutls)
 subdir('qapi')
 
 if have_tools
-  qsd_ss = qsd_ss.apply(config_targetos, strict: false)
+  qsd_ss = qsd_ss.apply({})
   qsd = executable('qemu-storage-daemon',
                    qsd_ss.sources(),
                    dependencies: qsd_ss.dependencies(),