Fix Windows build without --enable-distro-toolchain

[snowleopard]

UPDATE: I fixed the issue below by passing --enable-distro-toolchain to configure: #565. We need to make it possible to build Hadrian without --enable-distro-toolchain, but this has lower priority.

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Now hitting this error on Windows:

/---------------------------------------------------------------------------------\
| Successfully built program 'ghc-pkg' (Stage0).                                  |
| Executable: _build/stage0/bin/ghc-pkg.exe                                       |
| Program synopsis: A utility for querying and managing the GHC package database. |
\---------------------------------------------------------------------------------/
ghc.exe: could not execute: C:\msys\home\nam83\ghc\_build\stage0\lib/../mingw/bin/gcc.exe
ghc.exe: could not execute: C:\msys\home\nam83\ghc\_build\stage0\lib/../mingw/bin/gcc.exe
shakeArgsWith     0.001s    0%
Function shake   13.529s    0%
Database read     0.000s    0%
With database     0.000s    0%
Running rules  1780.870s   99%  =========================
Total          1794.401s  100%
Error when running Shake build system:
* _build/stage1/lib/package.conf.d/rts-1.0.conf
* OracleQ (PackageDataFile (Context {stage = Stage1, package = Package {pkgLanguage = C, pkgType = Library, pkgName = "rts", pkgPath = "rts"}, way = v}))
* _build/stage1/rts/setup-config
ExitFailure 1

[snowleopard]

The above commit works on my machine -- will close this if AppVeyor succeeds too.

[snowleopard]

🎉