Provide way to build using existing C compiler on Windows.
Summary: 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. Test Plan: ./configure --enable-distro-toolchain && make - && make THREADS=9 test ./validate Reviewers: austin, hvr, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie, erikd, #ghc_windows_task_force GHC Trac Issues: #13792 Differential Revision: https://phabricator.haskell.org/D3637
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