Fix LibMyPaint directory lookup in macOS
This MR derives from my work in !581 (merged). It fixes linking against libMyPaint in macOS.
cc @ivany
Test Plan
Build Krita in macOS.
Formalities Checklist
-
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.