Jrobinson/configure for cross

This request includes the fixes for the bashisms already reported plus a change which gives an escape route when targethost is not defined but usecrosscompile is defined - this allows for cross-builds to use your code to generate a host miniperl whilst continuing with provided config.h and config.sh files in order to cope with new architectures and architectures where no suitable targethost is viable.

I'll feed this back to you on the list too.