make sdist is broken, fails to generate source archive

Summary

The command make sdist-ghc fails after a fresh git repo clone (GHC master).

After a successfull GHC build the source dist build fails:

make sdist-ghc
make --no-print-directory -f ghc.mk sdist-ghc NO_INCLUDE_DEPS=YES NO_INCLUDE_PKGDATA=YES
ghc.mk:1221: warning: overriding recipe for target 'sdist_compiler_stage2_Cmm'
ghc.mk:1220: warning: ignoring old recipe for target 'sdist_compiler_stage2_Cmm'
make[1]: *** No rule to make target 'compiler/stage2/build/Cmm.hs', needed by 'sdist_compiler_stage2_Cmm'. Stop.
Makefile:162: recipe for target 'sdist-ghc' failed
make: *** [sdist-ghc] Error 2

Steps to reproduce

1. clone GHC git repository: git clone --recursive https://gitlab.haskell.org/ghc/ghc.git
2. ./boot
3. ./configure
4. make -j4 (optional, does not have effect on the problem)
5. make sdist-ghc

Expected behavior

A working source tar.xz should be generated and it should compile.

Could the source-dist generator be tested via CI?

Environment

• GHC version used: bootstrap GHC 8.6.2, GHC master git hash: 785008c1

Optional:

• Operating System: Ubuntu 16.04
• System Architecture: x64