Restore MSYS compatibility
This MR restores compatibility with MSYS2 toolchains, specifically CLANG64. It also adds a few notes on how to use those toolchains to build Krita.
Of special note are the changes to SIP and PyQt5; a reasonably updated setuptools is more than enough to build them, no need to hotpatch CygwinCCompiler anymore.
Test Plan
Build Krita with the standard and MSYS2 toolchains.
Formalities Checklist
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I confirmed this builds. -
I confirmed Krita ran and the relevant functions work. -
I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!) -
I made sure my commits build individually and have good descriptions as per KDE guidelines. -
I made sure my code conforms to the standards set in the HACKING file. -
I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.