Win32: Fix sub make commands

`cmd.exe` built-in `cd` does not recognize slashes as path separators,
replace to backslashes.

ext/extmk.rb

@@ -817,9 +817,9 @@ begin
     if $gnumake == "yes"
       submake = "$(MAKE) -C $(@D)"
     else
-      submake = "cd $(@D) && "
-      config_string("exec") {|str| submake << str << " "}
-      submake << "$(MAKE)"
+      submake = ["cd", (sep ? "$(@D:/=#{sep})" : "$(@D)"), "&&"]
+      config_string("exec") {|str| submake << str}
+      submake = (submake << "$(MAKE)").join(" ")
     end
     targets.each do |tgt|
       exts.each do |d|

template/exts.mk.tmpl

@@ -110,12 +110,14 @@ def self.column
   w
 end
 objext = RbConfig::CONFIG["OBJEXT"]
+sep = RbConfig::CONFIG['BUILD_FILE_SEPARATOR']
+sep = nil if sep and sep.empty?
 if gnumake
   submake = "$(MAKE) -C $(@D)"
 else
-  submake = "cd $(@D) && "
-  exec = RbConfig::CONFIG["exec"] and !exec.empty? and submake << exec << " "
-  submake << "$(MAKE)"
+  submake = ["cd", (sep ? "$(@D:/=#{sep})" : "$(@D)"), "&&"]
+  exec = RbConfig::CONFIG["exec"] and !exec.empty? and submake << exec
+  submake = (submake << "$(MAKE)").join(" ")
   mflags = " $(MFLAGS)"
 end
 -%>