This was done for C++ by commit v3.5.0-rc1~69^2 (Record compile features
for MinGW Clang on Windows, 2016-01-11).  Make the same change for C.
The `UNIX` condition on Clang C compiler features was already dropped by
refactoring in commit v3.9.0-rc1~17^2~4 (Compilers: Port to use default
cmake_record_lang_compile_features macros, 2017-05-10).

Our documentation already claims support for this combination.  This
was simply an oversight when support was added for MinGW Clang C++.

Issue: #15897
Issue: #15943