Gentoo has a Qt5Quick cmake module instead of a Qt5Declarative one.

--- luminance-hdr-2.4.0/CMakeLists.txt
+++ luminance-hdr-2.4.0/CMakeLists.txt
@@ -22,7 +22,7 @@
 find_package(Qt5Concurrent REQUIRED)
 find_package(Qt5Widgets REQUIRED)
 find_package(Qt5Gui REQUIRED)
-find_package(Qt5Declarative REQUIRED)
+find_package(Qt5Quick REQUIRED)
 find_package(Qt5WebKit REQUIRED)
 find_package(Qt5WebKitWidgets REQUIRED)
 find_package(Qt5Xml REQUIRED)