Check for jruby.sh on install and warn or error

The skinny launcher on unix requires that JRuby's jruby.sh be
present in the target installation. If it is not, we try to log
a warning to /dev/tty indicating that the native executable will
not be installed. If we cannot write to /dev/tty, we log the same
warning but fail the extconf build.

The warning explains the situation, links to the ruby-build issue
about deleting jruby.sh, and recommends upgrading the host Ruby
manager.

Author: headius

extconf.rb

@@ -1,3 +1,29 @@
+# check for jruby.sh used by new unix launcher
+if !File.exist?(File.join(RbConfig::CONFIG['bindir'], "jruby.sh"))
+  require 'mkmf'
+  error = <<~MSG
+    WARNING: This JRuby install does not contain bin/jruby.sh and is not compatible
+    WARNING: with jruby-launcher 2.0+.
+    WARNING:
+    WARNING: If you are using ruby-build to install (or a Ruby manager like rbenv)
+    WARNING: please upgrade to latest and reinstall jruby-launcher.
+    WARNING:
+    WARNING: See https://github.com/rbenv/ruby-build/pull/2517 for more details.
+  MSG
+  if File.exist?("/dev/tty")
+    error << <<~MSG
+      WARNING:
+      WARNING: The native jruby executable has not been installed.
+    MSG
+    File.write("/dev/tty", error)
+  else
+    $stderr.write(error)
+  end
+  File.write("Makefile", dummy_makefile($srcdir).join(""))
+  return
+end
+
+# jruby.sh exists, proceed to build
 mf = File.read('Makefile')
 mf = mf.gsub(/^BINDIR\s*=.*$/, "BINDIR = #{RbConfig::CONFIG['bindir']}")
 mf = mf.gsub(/^PREFIX\s*=.*$/, "PREFIX = #{File.dirname(RbConfig::CONFIG['libdir'])}")