Fixes GTK detection when using system wxWidgets #657

Job	Run time
Build macOS (arm64)	37m 16s
Build macOS (x86_64)	36m 8s
Ubuntu 20.04 AppImage (x86_64)	4m 52s
Ubuntu 20.04 AppImage (x86_64, gcc 11)	5m 2s
Ubuntu 22.04 AppImage (x86_64)	5m 11s
Windows MSVC 2019 (x86)	26m 14s
Source Tarball	5m 54s
Windows MSVC 2019 (x86_64)	38m 1s
Windows MSVC 2022 (x86)	27m 19s
Windows MSVC 2022 (x86_64)	25m 38s
Package macOS (x86_64, arm64, universal)	5m 51s
	3h 37m 26s

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