GNUmakefile.in: fix MSYS2_ARG_CONV_EXCL
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
GNUmakefile.in: fix MSYS2_ARG_CONV_EXCL
should be converted.
[ruby-dev:49526] [Bug #12199]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e