setup: fix installing in a mingw environment
The addition, in
9a4adc76c88a, of a hack for the MSVC compiler class was
overwriting the original class for the Mingw32CCompiler class, leading to an
error when the HackedMingw32CCompiler is instantiated.
Differential Revision: https://phab.mercurial-scm.org/D329
(binary:application/octet-stream)