Make sure bintest only works with host target.

lib/mruby/build.rb

@@ -196,6 +196,7 @@ EOS
     end
 
     def enable_bintest
+      raise "bintest works only on 'host' target" unless name == "host"
       @enable_bintest = true
     end
 

target/boxing.rb

@@ -7,7 +7,6 @@ MRuby::Build.new('no_boxing') do |conf|
 
   conf.gembox 'default'
   conf.enable_test
-  conf.enable_bintest
 end
 
 MRuby::Build.new('word_boxing') do |conf|
@@ -18,7 +17,6 @@ MRuby::Build.new('word_boxing') do |conf|
     c.defines += %w(MRB_WORD_BOXING)
   end
   conf.enable_test
-  conf.enable_bintest
 end
 
 MRuby::Build.new('nan_boxing') do |conf|
@@ -29,6 +27,5 @@ MRuby::Build.new('nan_boxing') do |conf|
     c.defines += %w(MRB_NAN_BOXING)
   end
   conf.enable_test
-  conf.enable_bintest
 end
 

target/host-debug.rb

@@ -20,5 +20,4 @@ MRuby::Build.new('host-debug') do |conf|
   conf.gem :core => "mruby-bin-debugger"
 
   # bintest
-  # conf.enable_bintest
 end

target/travis.rb

@@ -31,7 +31,6 @@ MRuby::Build.new('cxx_abi') do |conf|
   conf.compilers.each do |c|
     c.defines += %w(MRB_GC_FIXED_ARENA)
   end
-  conf.enable_bintest
   conf.enable_test
 
   enable_cxx_abi