Make-based build fails on Windows with pre-existing MSYS2/MinGW toolchain
# Summary
This is related to https://gitlab.haskell.org/ghc/ghc/issues/15948
Make-based build fails on Windows with pre-existing MSYS2/MinGW toolchain. So pacman MinGW package can't be created.
# Steps to reproduce
1. Install MSYS/MinGW toolchain
2. Install packages base-devel msys2-devel mingw-w64-i686-toolchain mingw-w64-x86_64-toolchain
3. Install dependencies from provided PKGBUILD through pacman
```
makedepends=("${MINGW_PACKAGE_PREFIX}-python3-sphinx"
"${MINGW_PACKAGE_PREFIX}-llvm"
'make'
'perl')
optdepends=("${MINGW_PACKAGE_PREFIX}-llvm: use LLVM for an optimized backend")
```
2. `git clone https://github.com/msys2/MINGW-packages.git`
3. `cd MINGW-packages/mingw-w64-ghc`
4. `makepkg-mingw -sCLf --skippgpcheck`
Result:
[mingw-w64-ghc-8.6.5-1-x86_64-build.log](/uploads/5f61d5e0b69b7781adea6b7c1dc0c601/mingw-w64-ghc-8.6.5-1-x86_64-build.log)
`ghc-stage1.exe: could not detect mingw toolchain`
# Environment
* GHC version used: 8.6.5 (still not fixed in 8.10.1 too)
Optional:
* Operating System: Windows 8.1 Enterprise 64-bit
* System Architecture:
P.S. This issue doesn't related to Happy and Alex issues anymore. See logs.
issue