examples: fix "call v from ruby" example for Windows and Mac (#20487)

Author: omnikron

examples/call_v_from_ruby/test.rb

@@ -7,10 +7,26 @@
 
 require 'ffi'
 
+# extension for shared libraries varies by platform - see vlib/dl/dl.v
+# get_shared_library_extension()
+def shared_library_extension
+  if Gem.win_platform?
+    '.dll'
+  elsif RUBY_PLATFORM =~ /darwin/ # MacOS
+    '.dylib'
+  else
+    '.so'
+  end
+end
+
 module Lib
   extend FFI::Library
 
-  ffi_lib File.join(File.dirname(__FILE__), 'test.so')
+  begin
+    ffi_lib File.join(File.dirname(__FILE__), 'test' + shared_library_extension)
+  rescue LoadError
+    abort("No shared library test#{shared_library_extension} found. Check examples/call_v_from_ruby/README.md")
+  end
 
   attach_function :square, [:int], :int
   attach_function :sqrt_of_sum_of_squares, [:double, :double], :double