Specify receivers for gembox methods; ref #5241

mrbgems/default-no-fpu.gembox

@@ -1,3 +1,3 @@
 MRuby::GemBox.new do |conf|
-  gembox "stdlib"
+  conf.gembox "stdlib"
 end

mrbgems/default-no-stdio.gembox

@@ -1,4 +1,4 @@
 MRuby::GemBox.new do |conf|
-  gembox "stdlib"
-  gembox "math"
+  conf.gembox "stdlib"
+  conf.gembox "math"
 end

mrbgems/default.gembox

@@ -1,9 +1,9 @@
 MRuby::GemBox.new do |conf|
-  gembox "stdlib"
-  gembox "stdlib-ext"
-  gembox "stdlib-io"
-  gembox "math"
-  gembox "metaprog"
+  conf.gembox "stdlib"
+  conf.gembox "stdlib-ext"
+  conf.gembox "stdlib-io"
+  conf.gembox "math"
+  conf.gembox "metaprog"
 
   # Generate mrbc command
   conf.gem :core => "mruby-bin-mrbc"