fail to build on windows after #77 (Add UnicodeCLib)

[notch1p]

currently when compiling on Windows the project would fail due to linker error (Log below)

✖ [13/15] Building UnicodeBasic.TableLookup:dynlib
trace: .> c:\Users\notch1p\.elan\toolchains\leanprover--lean4---v4.22.0\bin\clang.exe -shared -o C:\Us
ers\notch1p\Downloads\lean4-unicode-basic\.lake\build\lib\lean\UnicodeBasic_TableLookup.dll @C:\Users\
notch1p\Downloads\lean4-unicode-basic\.lake\build\lib\lean\UnicodeBasic_TableLookup.dll.rsp
info: stderr:
ld.lld: error: undefined symbol: unicode_case_lookup
>>> referenced by C:\Users\notch1p\Downloads\lean4-unicode-basic\.lake\build\ir\UnicodeBasic\TableLook
up.c.o.export:(l_Unicode_CLib_lookupCase___boxed)
>>> referenced by C:\Users\notch1p\Downloads\lean4-unicode-basic\.lake\build\ir\UnicodeBasic\TableLook
up.c.o.export:(l_Unicode_lookupCaseMapping)

ld.lld: error: undefined symbol: unicode_prop_lookup
>>> referenced by C:\Users\notch1p\Downloads\lean4-unicode-basic\.lake\build\ir\UnicodeBasic\TableLook
up.c.o.export:(l_Unicode_CLib_lookupProp___boxed)
>>> referenced by C:\Users\notch1p\Downloads\lean4-unicode-basic\.lake\build\ir\UnicodeBasic\TableLook
up.c.o.export:(l_Unicode_lookupGC)
>>> referenced by C:\Users\notch1p\Downloads\lean4-unicode-basic\.lake\build\ir\UnicodeBasic\TableLook
up.c.o.export:(l_Unicode_lookupGC___boxed)
>>> referenced 16 more times
clang: error: linker command failed with exit code 1 (use -v to see invocation)
error: external command 'c:\Users\notch1p\.elan\toolchains\leanprover--lean4---v4.22.0\bin\clang.exe' 
exited with code 1
Some required builds logged failures:
- UnicodeBasic.TableLookup:dynlib
error: build failed

I suspect this has something to do with the UnicodeCLib we're currently using which uses CFFI. After switching to a commit before the addition of such library I can confirm that it builds successfully. But the reason that this would only fail on Windows is unclear to me: Maybe because ld can't find the static unicodeclib.a specifically on Windows? -- But I'm not sure.

So I compared the current lakefile to one that a project of mine are currently using (which also uses CFFI but can build on Windows), and it turns out that what's missing is a extern_lib declaration:

extern_lib libunicodeclib := UnicodeCLib.fetch

after adding the above we can successfully build on windows and this modification doesn't affect existing builds on other platforms (See workflow).

I've made a PR to fix this, see #82.

[fgdorais]

I was worried this would happen! Thanks for the report!

[fgdorais]

Investigation is ongoing. This may be a Lake bug.