MSVC: Don't warn when projects add /Wall flag

[drobilla]

See commit message for details.

[jpakkane]

Looking at MSDN docs, /Wall is a setting that has even more stuff than /W4, meaning it really should be called /W5 for consistency. That would break things, since we specify that warning_level 4 means "everything". sigh Why does everything have to be needlessly complicated?

Could we add the value everything to warning level instead that would explicity mean absolutely everything on every compiler?

[drobilla]

Could we add the value everything to warning level instead that would explicitly mean absolutely everything on every compiler?

I hope so (where "absolutely everything" is a little bit tricky but doable, I think). I'm going to take a stab at that as well, this was just some low-hanging fruit. That will be a bit of work, though, current meson warning levels are nowhere near everything for any compiler.

[codecov]

Codecov Report

Merging #7969 (1b86381) into master (b40e4de) will decrease coverage by 1.31%.
The diff coverage is n/a.

❗ Current head 1b86381 differs from pull request most recent head 10b0fe6. Consider uploading reports for the commit 10b0fe6 to get more accurate results

@@            Coverage Diff             @@
##           master    #7969      +/-   ##
==========================================
- Coverage   68.59%   67.28%   -1.32%     
==========================================
  Files         412      207     -205     
  Lines       87867    44928   -42939     
  Branches    20728     9280   -11448     
==========================================
- Hits        60271    30228   -30043     
+ Misses      23088    12419   -10669     
+ Partials     4508     2281    -2227     

Impacted Files	Coverage Δ
modules/cuda.py	0.00% <0.00%> (-72.64%)	⬇️
templates/cudatemplates.py	37.50% <0.00%> (-62.50%)	⬇️
compilers/cuda.py	19.63% <0.00%> (-45.40%)	⬇️
dependencies/cuda.py	20.19% <0.00%> (-42.79%)	⬇️
compilers/mixins/clang.py	52.87% <0.00%> (-13.80%)	⬇️
scripts/coverage.py	56.43% <0.00%> (-7.93%)	⬇️
compilers/detect.py	42.97% <0.00%> (-3.63%)	⬇️
linkers/linkers.py	56.35% <0.00%> (-1.25%)	⬇️
environment.py	80.12% <0.00%> (-1.08%)	⬇️
dependencies/misc.py	44.55% <0.00%> (-0.75%)	⬇️
... and 209 more

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[drobilla]

Supplanted by #10554

I guess I'll leave this open until the issue is resolved on master one way or another.