meson: Display libfdt as disabled when system emulation is disabled

When configuring QEMU with --disable-system, meson keeps showing
libfdt as "auto". Mark it as disabled instead.

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220214183144.27402-2-f4bug@amsat.org>

meson.build

@@ -2432,8 +2432,8 @@ if get_option('cfi') and slirp_opt == 'system'
 endif
 
 fdt = not_found
-fdt_opt = get_option('fdt')
 if have_system
+  fdt_opt = get_option('fdt')
   if fdt_opt in ['enabled', 'auto', 'system']
     have_internal = fs.exists(meson.current_source_dir() / 'dtc/libfdt/Makefile.libfdt')
     fdt = cc.find_library('fdt', kwargs: static_kwargs,
@@ -2476,6 +2476,8 @@ if have_system
     fdt = declare_dependency(link_with: libfdt,
                              include_directories: fdt_inc)
   endif
+else
+  fdt_opt = 'disabled'
 endif
 if not fdt.found() and fdt_required.length() > 0
   error('fdt not available but required by targets ' + ', '.join(fdt_required))