meson.build: Do not look for VNC-related libraries if have_system is not set

When running "./configure --static --disable-system" there is currently
a warning if the static version of libpng is missing:

 WARNING: Static library 'png16' not found for dependency 'libpng', may not
 be statically linked

Since it does not make sense to look for the VNC-related libraries at all
when we're building without system emulator binaries, let's add a check
for have_system here to silence this warning.

Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: huth

meson.build

@@ -931,7 +931,7 @@ vnc = not_found
 png = not_found
 jpeg = not_found
 sasl = not_found
-if not get_option('vnc').disabled()
+if have_system and not get_option('vnc').disabled()
   vnc = declare_dependency() # dummy dependency
   png = dependency('libpng', required: get_option('vnc_png'),
                    method: 'pkg-config', kwargs: static_kwargs)