According to the [Meson log](https://github.com/geany/geany/files/11058531/meson-log.txt), it's calling `cc`, which I believe is just a copy of the `gcc` executable:
``` $ which cc /mingw64/bin/cc
$ ls -l /mingw64/bin/[^g]*cc.exe -rwxr-xr-x 1 antum antum 2350907 Jan 23 00:22 /mingw64/bin/x86_64-w64-mingw32-cc.exe -rwxr-xr-x 1 antum antum 2350907 Jan 23 00:22 /mingw64/bin/x86_64-w64-mingw32-gcc.exe ```
It also defines `WIN32` as `1`.