Let users disable environment entries

It's helpful to be able to temporarily disable environment variables, as
opposed to having to remove (and then re-add) them entirely.

Fixes: QTCREATORBUG-20984
Change-Id: Ib0d287035b9357507c4c19faaf3a1517382506b5
Reviewed-by: hjk <hjk@qt.io>

src/app/main.cpp

@@ -408,7 +408,7 @@ int main(int argc, char **argv)
                 {{"LD_LIBRARY_PATH", "", Utils::EnvironmentItem::Unset}});
         } else {
             Utils::Environment::modifySystemEnvironment(
-                {{"LD_LIBRARY_PATH", *options.userLibraryPath, Utils::EnvironmentItem::Set}});
+                {{"LD_LIBRARY_PATH", *options.userLibraryPath, Utils::EnvironmentItem::SetEnabled}});
         }
     }