JT: Add env var $JT_JDK to select the JDK globally

Author: eregon

tool/jt.rb

@@ -741,6 +741,8 @@ def help
         OPENSSL_PREFIX                               Where to find OpenSSL headers and libraries
         ECLIPSE_EXE                                  Where to find Eclipse
         SYSTEM_RUBY                                  The Ruby interpreter to run 'jt' itself, when using 'bin/jt'
+        JT_JDK                                       The JDK version to use: 8 or 11 (default)
+        JT_PROFILE_SUBCOMMANDS                       Print the time each subprocess takes on stderr
     TXT
   end
 
@@ -2706,7 +2708,7 @@ def process_pre_args(args)
     needs_build = false
     needs_rebuild = false
     @silent = false
-    @jdk_version = 11
+    @jdk_version = Integer(ENV['JT_JDK'] || 11)
 
     until args.empty?
       arg = args.shift