https://github.com/PCSX2/pcsx2/commit/6386be09c2e disabled PACKAGE_MODE
for reasons that make no sense as far as installing from an ebuild goes.

Skip the error and handle the rest from the ebuild.
--- a/pcsx2-qt/CMakeLists.txt
+++ b/pcsx2-qt/CMakeLists.txt
@@ -7,3 +7,2 @@
 if (PACKAGE_MODE)
-	message(FATAL_ERROR "Package mode is not supported for Qt builds.")
 else()