Fix test syntax in configure.in

Commits 1e4bdcfe and 8df1965d used the wrong test equality operator.
Some versions of test allow == for equality, but others (such as the
HP-UX version) do not.  Use a single = for correctness.

[ghudson@mit.edu: clarified commit message]

ticket: 7992 (new)
target_version: 1.13
tags: pullup