Clear `DESTDIR` when running make

* spec/rubyspec/optional/capi/spec_helper.rb (compile_extension): `DESTDIR` is
  the drive letter of the ruby installed path as default on mswin, but not
  builddir of the ruby.  this causes spec errors if the drive letter is
  different in the installed path and builddir.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59096 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/rubyspec/optional/capi/spec_helper.rb

@@ -59,7 +59,7 @@ def compile_extension(name)
       ENV.delete "MAKEFLAGS" # Fix make warning when invoked with -j in MRI
 
       # Do not capture stderr as we want to show compiler warnings
-      output = `#{make} V=1`
+      output = `#{make} V=1 DESTDIR=`
       raise "#{make} failed:\n#{output}" unless $?.success?
       $stderr.puts output if debug