fix missing solvers

Fix #349 
The error of 'snopt not found' is only thrown if there is no lhs for
checkDependency function. So snopt_enabled = checkDependency('snopt') will
silent the error in compareSolvers.


On Thu, Aug 28, 2014 at 4:03 PM, RussTedrake <notifications@github.com>
wrote:

> In solvers/NonlinearProgram.m:
>
> > @@ -878,10 +878,12 @@
> >      function [x,objval,exitflag,execution_time] = compareSolvers(obj,x0,solvers)
> >        if nargin<3
> >          solvers = {'fmincon'};
> > -        if(checkDependency('snopt'))
> > +        snopt_enabled = checkDependency('snopt');
> > +        if(snopt_enabled)
>
> how does this change the behavior in any way?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/RobotLocomotion/drake/pull/358/files#r16864239>.
>