Compute $(STDLIB_MODULES) with GNU make functions instead of runtime/sak

[dra27]

This PR follows on from #12287 (comment).

stdlib/StdlibModules contains the macro $(STDLIB_MODULES) which expands to the actual module list for the Standard Library (i.e. with Stdlib__ prefixes). In #10169, this was computed using a series of shell calls which was hideously slow on Windows and which got changed in #10451 with the introduction of the sak tool.

Even ignoring questions of taste or otherwise about the use of C, this constitutes a slightly confusing state of affairs in the build system. $(STDLIB_MODULES) is evaluated, but it is the responsibility of the expansion site to ensure that the sak tool is compiled before this happens. In other words, the STDLIB_MODULES variable has prerequisites!

This isn't ideal, and means that Makefile.common contains some dark magic to ensure that sak is compiled before StdlibModules is included (which can also cause sak to be (re-)built when not really necessary).

This can be done fairly simply, if inelegantly, in GNU make itself. GNU make doesn't directly have a function to convert case, but it can efficiently evaluate 26 calls to $(patsubst ... - i.e. we can just do $(patsubst a%, A%, $(patsubst b%, B%, .... I'm not that keen on 27 right brackets in a row at the end of it, so instead the macro CAPITALIZE here receives two space-separated alphabets, and is defined recursively. I like this not involving any auxiliary script.

runtime/sak remains only for the purpose of formatting the location of the Standard Library, but this is used in a much more normal way to generate runtime/build_config.h (where runtime/sak is just a normal dependency). I think I have something in the freezer for that, too, but for another day.

[shindere]

The PR definitely seems sensible to me. My only point of scepticism at that point is that I find the documentation for `CAPITALIZE` not very clear, because the comment mentions 5 parameters which do not appear in the subsequentOCaml code so I wonder if this OCaml equivalent really helps. Well to be honest I for myself found it more confusing than enlightening.

[nojb]

LGTM

Please leave the OCaml analog in; it helps understanding the macro for Make non-experts.

[dra27]

Thanks, @nojb! @shindere and I ran out of steam in the heat yesterday, but there’s a slightly simpler version at dra27@e307048#diff-0ed518c14a9f4fdaa0f4d9fb9680bf7c1d4068015e46112d39e735c53cace4dfR107 (with a non-OCaml comment) - how does that look to you?

[nojb]

how does that look to you?

Also LGTM!

[dra27]

Thanks! Revised version pushed here - I'll leave it for Monday when @shindere will have a chance to have a look.

[shindere]

I like the current version very much! Many thanks for your patience, @dra27!