Fix some duplicate code in configure.ac

This is a couple of minor fixes for configure.ac.

The first is to remove a block that checks the $gldispatch_use_tls variable before the variable is assigned. That block is also a duplicate of one later in the file, which is where the check is supposed to be. The first one ends up being a no-op.

The second change is just combining a couple of branches in a case block that don't need separate handling.