Don't attempt to build benchmarks if release-mode Qt is not available

Don't attempt to build benchmarks if release-mode Qt is not available

All of the benchmarks explicitly turn on release mode.  This fails on
Mac and Windows if Qt is configured as debug-only.  It is also possibly
misleading on Linux, as compiling the benchmark in release mode does
not imply that Qt is compiled in release mode.

The results are generally not useful if release mode Qt is not
available, so disable them by default in this case.

Change-Id: Ifa2bb6ab6412dd360ff9dfb890ace2f51e0eef86
Reviewed-on: http://codereview.qt.nokia.com/549
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jo Asplin <jo.asplin@nokia.com>