Fix compilation with libidn2 runtime on Windows

[equeim]

[neheb]

how is libidn2 added? vcpkg? I don't see this issue under MSYS2:

https://github.com/neheb/wrapdb/pull/4

[rockdaboot]

how is libidn2 added? vcpkg? I don't see this issue under MSYS2:

neheb/wrapdb#4

We possibly need something slightly amended, like the first answer in https://stackoverflow.com/questions/3694723/error-c3861-strcasecmp-identifier-not-found-in-visual-studio-2008

[rockdaboot]

Thanks for your contribution. It made me take another look at the use of strcasecmp. And it was just used to check if a string is not utf-8. I just pushed a hand-rolled version of this test to increase portability.