Enable compiler optimization by default

At present, Mesos defaults to compiling with "-O0"; to enable compiler
optimizations, the user needs to specify "--enable-optimize" when running configure.

We should change the default for the following reasons:

(1) The autoconf default for CFLAGS/CXXFLAGS is "-O2 -g". Anecdotally,
I think most software packages compile with a reasonable level of
optimizations enabled by default.

(2) I think we should make the default configure flags appropriate for
end-users (rather than Mesos developers): developers will be familiar
enough with Mesos to tune the configure flags according to their own
preferences.

(3) The performance consequences of not enabling compiler
optimizations can be pretty severe: 5x in a benchmark I just ran, and
we've seen between 2x and 30x performance differences for some
real-world workloads.