Fix EXECUTABLE_SUFFIX handling.
Fix EXECUTABLE_SUFFIX handling.

It was broken because of the way variables where handled. Differentiate between
path variables and other variables.

TBR=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/11414087

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@168845 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py