Make it easier to have multiple executables

[litchfld]

At the moment main.o is treated on the same footing as all other object files which is not ideal if you want to be able to build multiple applications, since all applications must define an int main(argc argv). It can be worked round but it's a bit of a nuisance.

This showed up while I was trying to build a test harness, but it would be useful for other cases: building more complete development/regression tests; subsidary executables that create template files; etcetc...

[AndrewEdmonds11]

This has been done right, Phill?

[litchfld]

Nope. I have hacked the makefile about a bit to make the unit test executable, but nothing systematic. If we starting wanting a particular executable for calibration- or template-generating processes it might be helpful, but perhaps that just corresponds to specific card files.