meson.build: Don't use internal libfdt if the user requested the system libfdt

If the users ran configure with --enable-libfdt=system, they likely did
that on purpose. We should not silently fall back to the internal libfdt
if the system libfdt is not usable, but report the problem with a proper
message instead.

Message-Id: <20210827120901.150276-3-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

meson.build

@@ -1912,6 +1912,8 @@ if have_system
        int main(void) { fdt_check_full(NULL, 0); return 0; }''',
          dependencies: fdt)
       fdt_opt = 'system'
+    elif fdt_opt == 'system'
+       error('system libfdt requested, but it is too old (1.5.1 or newer required)')
     elif have_internal
       fdt_opt = 'internal'
     else