Ensure libv8 is required when rubygems is not installed

Do not break the build process if rubygems cannot be required.
Instread try to require libv8 without selecting a version with gem
and warn the user about it. This is necessary as ruby 1.8 does not
ship with rubygems by default.

Author: ignisf

ext/v8/extconf.rb

@@ -16,9 +16,18 @@
   $CFLAGS += " -O0 -ggdb3"
 end
 
-require 'rubygems'
-gem 'libv8', '~> 3.15.11'
-require 'libv8'
+LIBV8_COMPATIBILITY = '~> 3.15.11'
+
+begin
+  require 'rubygems'
+  gem 'libv8', LIBV8_COMPATIBILITY
+rescue Gem::LoadError
+  warn "Warning! Selecting libv8 #{LIBV8_COMPATIBILITY} failed. Has it been added to the gemspec?"
+rescue LoadError
+  warn "Warning! Could not load rubygems. Please make sure you have libv8 #{LIBV8_COMPATIBILITY} installed."
+ensure
+  require 'libv8'
+end
 
 Libv8.configure_makefile