1. Apparatus for generating a group of machine code entities for execution upon a target data processing apparatus from a group of source code entities, each source code entity having at least one build option parameter, said apparatus for generating comprising: (i) library selector forming logic responsive to said at least one build option parameter of each source code entity for forming a library selector, wherein: (a) build option parameters of each source code entity are combined in accordance with lattice theory whereby each build option parameter has a value indicative of position within a partially ordered lattice chain representing relative execution environment requirements for that build option parameter, and (b) said library selector is formed with components corresponding to a limiting level of execution environment requirements among said group of source code entities; (ii) library selecting logic responsive to said library selector for selecting, from among a plurality of libraries of machine code entities, a selected library of machine code entities having a best level of execution environment requirements compatible with said limiting level of execution environment requirements indicated by said library selector; and (iii) machine code entity generating logic for generating said group of machine code entities from machine code entities within said selected library of machine code entities in dependence upon said group of source code entities