Could not parse vswhere.exe output when pip install contourpy in msys2 mingw64 environment

[f-zl]

To reproduce:
In msys2 mingw64 environment, do
python -m venv test && cd test
source bin/activate
pip install contourpy
I've having this problem when installing matplotlib, which depends on contourpy.

Error message is:

• meson setup D:/Program/msys64/tmp/pip-install-srfkui18/contourpy_aac1892589db478c9ae47af268e4e920 D:/Program/msys64/tmp/pip-install-srfkui18/contourpy_aac1892589db478c9ae47af268e4e920/.mesonpy-kcb4fvc_ -Dbuildtype=release -Db_ndebug=if-release -Db_vscrt=md --vsenv --native-file=D:/Program/msys64/tmp/pip-install-srfkui18/contourpy_aac1892589db478c9ae47af268e4e920/.mesonpy-kcb4fvc_/meson-python-native-file.ini
  The Meson build system
  Version: 1.9.1
  Source dir: D:/Program/msys64/tmp/pip-install-srfkui18/contourpy_aac1892589db478c9ae47af268e4e920
  Build dir: D:/Program/msys64/tmp/pip-install-srfkui18/contourpy_aac1892589db478c9ae47af268e4e920/.mesonpy-kcb4fvc_
  Build type: native build

../meson.build:1:0: ERROR: Could not parse vswhere.exe output

I've tried setting CC and CXX environment variables but that doesn't work. How to make meson use mingw64 gcc toolchain? Thanks very much!