./configure && make -j4 is returning:

Failed to build these modules:
_tkinter                                                       

I'm running macOS 10.15.2, with the SDK installed using `xcode-select --install` (no funny business)

/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks/Tk.framework/

xcrun --show-sdk-path
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk

If I debug setup.py, I can see it's eagerly returning the wrong path (/System)

On a REPL, if I do:
>>> import setup
>>> setup.macosx_sdk_root()
'/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk'

So the path is correct, but I can see in setup.py that detect_tkinter_darwin() will break inside the loop if /System/Library/Frameworks/Tk.framework/Versions/Current  exists (which it does), but it doesn't contain the headers, it contains 3 directories:

Resources      Tk             _CodeSignature

I think detect_tkinter_darwin should be updated so that framework_dirs scans macosx_sdk_root first, but I don't know what other scenarios this might break.

I'd be happy to submit a patch for my scenario but it looks like this whole function needs tests so each platform change and scenario as they come up. There's loads of other issues on BPO related.