Provide way to build using existing C compiler on Windows.
There are various distros that build GHC using their own C compilers
such as MSYS2. Currently they have to patch the build scripts everytime.
This patch provides the configure argument --enable-distro-toolchain
which allows one to build using any C compiler on the path.
This is also useful for testing new versions of GCC.
./configure --enable-distro-toolchain && make - && make THREADS=9 test
Reviewers: austin, hvr, bgamari
Reviewed By: bgamari
Subscribers: rwbarton, thomie, erikd, #ghc_windows_task_force
GHC Trac Issues: Trac #13792
Differential Revision: https://phabricator.haskell.org/D3637