hw/display: avoid creating empty loadable modules

hw/display: avoid creating empty loadable modules

When using --disable-virglrenderer, QEMU still creates

  hw-display-virtio-gpu-gl.so
  hw-display-virtio-vga-gl.so
  hw-display-virtio-gpu-pci-gl.so

but when these are loaded, they provide no functionality as the code
which registers types is not compiled in. Funtionally this is
relatively harmless, because QEMU is fine loading a module with no
types.

This is rather confusing for users and OS distro maintainers though,
as they think they have the GL functionality built, but in fact the
module they are looking at provides nothing of value.

The root cause is the use of 'when/if_true' rules when adding sources
to the module source set. If all the rules evaluate to false, then we
have declared the module, but not added anything to it.  We need to
put declaration of the entire module inside a condition based on
existance of the 3rd party library deps that are mandatory.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1352
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20221219125830.2369169-1-berrange@redhat.com>
[Do not check for pixman. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>