Makefile: add 'clean_crystal' target

[makenowjust]

It is useful in compiler development cycle.

$ vim src/...  # edit compiler sources (but incomplete)
$ make crystal # oops! maked incompilable compiler...
$ make crystal # failed because compiler does not work

At such time I don't want to use make clean because it removes libcrystal and llvmext also. I don't want to build C++ files again, so I added clean-crystal target in Makefile. It removes the files built by crystal command. Now, we can use make clean_crystal crystal to recompile crystal after removing compiler itself.

[bcardiff]

I've done something like in my working copy with an additional phony rebuild as rebuild: clean_crystal crystal rule so I can do

$ make rebuild # compile deps and compiler
$ make rebuild # compile just the compiler

But I always wonder if it won't be better to clean_crystal as a dependency of crystal.

[RX14]

Compiling a stage 2 compiler with a compiled compiler is very useful (I did it a lot with #4182). clean_crystal shouldn't be a dependency of crystal.

[makenowjust]

@RX14 Yes. I think so too, and so I don't make clean_crystal as dependency of crystal.

[mverzilli]

I think this is useful, but I'm a bit worried about naming. Having clean, crystal and clean_crystal which is not exactly the addition of both seems like it will cause some confusion. Can we think of a more distinctive name? Even stage_2 is probably terrible but better than clean_crystal. Or is it just me?

[bcardiff]

@mverzilli the proposed clean_crystal is actually ensuring a stage_1 compiler since it removed the previously built compiler.

What about fresh_crystal?

[makenowjust]

I wrote description for clean_crystal: "Clean up crystal built files". It removes not only .build directory but also doc directory, and its name comes from abbreviation of clean_crystal_built_files.

[RX14]

I think clean_crystal is fine and we should merge this PR as-is, but fresh_crystal would be second best. The standard terminology in makefiles is clean though. fresh_crystal would probably mean fresh_crystal: clean_crystal crystal to me.

[mverzilli]

Fair enough, I'm probably the least experienced with make in this discussion. If this makes sense to all of you, I have no option but trust you :).