Unset LD_PRELOAD on mingw
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52558 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Unset LD_PRELOAD on mingw
crash at make test-all with LD_PRELOAD.
[ruby-core:71461] [Bug #11680]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52558 b2dd03c8-39d4-4d8f-98ff-823fe69b080e