Index: jruby-launcher/extconf.rb
--- jruby-launcher/extconf.rb.orig
+++ jruby-launcher/extconf.rb
@@ -1,6 +1,7 @@
 mf = File.read('Makefile')
 mf = mf.gsub(/^BINDIR\s*=.*$/, "BINDIR = #{RbConfig::CONFIG['bindir']}")
 mf = mf.gsub(/^PREFIX\s*=.*$/, "PREFIX = #{File.dirname(RbConfig::CONFIG['libdir'])}")
+mf = mf.gsub(/^INSTALLDIR\s*=.*$/, "INSTALLDIR = #{File.dirname(RbConfig::CONFIG['libdir'])}/lib/ruby/stdlib/rubygems/defaults")
 mf = mf.gsub(/^JRUBY_VERSION\s*=.*$/, "JRUBY_VERSION = #{JRUBY_VERSION}")
 
 # Launcher will use .module_opts file if present, otherwise hardcoded add-opens for this module.