Record C compile features flags for MinGW Clang on Windows
This was done for C++ by commit 3baca636. Make the same change for C. The UNIX
condition on Clang C compiler features was already dropped by refactoring in !833 (merged).
Our documentation already claims support for this combination. This was simply an oversight when support was added for MinGW Clang C++.
Issue: #15897 (closed)
Issue: #15943