tool/extlibs.rb: Use Exception#message for older rubies

This script runs with BASERUBY, which can be as old as 2.2.x.
Exception#full_message is new in 2.5.0.

I saw a NoMethodError on the AppVeyor CI where BASERUBY is 2.4.6:
https://ci.appveyor.com/project/ruby/ruby/builds/43870654/job/kae4uo2xbhuhqmdw#L121

tool/extlibs.rb

@@ -227,7 +227,7 @@ class ExtLibs
       begin
         extracted = do_command(mode, dest, url, cache_dir, chksums)
       rescue => e
-        warn e.full_message
+        warn defined?(e.full_message) ? e.full_message : e.message
         success = false
       end
       url = chksums = nil