Define NDEBUG when buildVariant is release

It is conventional for the build system to define this preprocessor macro when generating a release build [1][2]. QBS should therefore define it when buildVariant===release, to avoid the necessity for a user to add a block like this:

  Properties {
	condition: qbs.buildVariant === "release"
	cpp. defines: outer.concat (["NDEBUG"])
  }

[1] http://stackoverflow.com/questions/1878645/where-does-the-dndebug-normally-come-from
[2] http://stackoverflow.com/questions/3167161/when-does-gcc-define-ndebug