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