When one is not using the msys2 python, the header files that are passed in as environment
variable cannot be found.
https://ci.appveyor.com/project/fonttools/ttfautohint-py/build/1.0.65/job/rkremny4jjid9nl2#L803
This is because msys2 shell and make use POSIX paths (e.g. /c/Users/clupo/...)
whereas non-msys2 python.exe uses native Windows paths (e.g. C:\Users\clupo\...).
Msys2 will automatically convert command line arguments (but not environment variables) from
POSIX to Windows paths when calling a native win32 executable, so we pass the header paths
as arguments instead of environment variables.
This way the gen-def.py script can support both mingw python running in an MSYS2 shell, and
native win32 python.
Cosimo Lupo
Ebrahim Byagowi