Unify stack.yaml and stack-macos.yaml

[dan-zheng]

Context

stack-macos.yaml is a copy of stack.yaml, with one additional llvm-hs flag at the bottom:

dex-lang/stack-macos.yaml

Lines 24 to 26 in 269c26c

	flags:
	llvm-hs:
	shared-llvm: false

The flag seems necessary to prevent this crash during stack build:

$ stack build
llvm-hs> configure
llvm-hs> [1 of 2] Compiling Main             ( /private/var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/stack-faa6f21d644c0b9d/llvm-hs-9.0.1/Setup.hs, /private/var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/stack-faa6f21d644c0b9d/llvm-hs-9.0.1/.stack-work/dist/x86_64-osx/Cabal-3.0.1.0/setup/Main.o )
llvm-hs> [2 of 2] Compiling StackSetupShim   ( /Users/danielzheng/.stack/setup-exe-src/setup-shim-mPHDZzAJ.hs, /private/var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/stack-faa6f21d644c0b9d/llvm-hs-9.0.1/.stack-work/dist/x86_64-osx/Cabal-3.0.1.0/setup/StackSetupShim.o )
llvm-hs> Linking /private/var/folders/m_/6f7q8zfs3n9fr0c_4gy8840m00hc_q/T/stack-faa6f21d644c0b9d/llvm-hs-9.0.1/.stack-work/dist/x86_64-osx/Cabal-3.0.1.0/setup/setup ...
llvm-hs> setup: '/usr/local/opt/llvm@9/bin/llvm-config' exited with an error:
llvm-hs> llvm-config: error: libLLVM-9.dylib is missing
llvm-hs>
Progress 1/2

Goal

Unifying the two stack.yaml files would be a big win for build simplification and maintainability.

A simple approach would be to add platform-specific configuration to dex.cabal or stack.yaml. HaskTorch friends (@junjihashimoto, @tscholak) shared that Stack does not support platform-specific configuration, but Cabal does appear to support it.

[apaszke]

This is already tracked in commercialhaskell/stack#2048 and commercialhaskell/stack#3369. We can't fix that yet.