makeSLProgram -- create a straight-line program (either interpreted or compiled)

Passes the arguments to makeInterpretedSLProgram or makeCompiledSLProgram (missing documentation) depending on the flag set by setTryJustInTimeCompilation. (Giving `true` as a value triggers a search for gnu C++ compiler and, if successful, makes a makeCompiledSLProgram (missing documentation) the default choice. Currently, just-in-time compilation works over RR_53 and CC_53)