Fix the shell operator bug of "==" to "=" in erts/configure.in

[Kenji Rikitake]

git fetch git://github.com/jj1bdx/otp.git r13b03_erts_configure

In UNIX test command, "==" is incorrect for string comparison;
"=" is the right operator.  Originally pointed out by
Giacomo Olgeni in the following article:
http://www.erlang.org/pipermail/erlang-patches/2008-November/000334.html
and available as a FreeBSD port patch.

Accordingly fixed in erts/configure.in

Regards,
Kenji Rikitake