Sadly the CMake side of openjpeg slotting in Gentoo is broken.
To reliably find openjpeg:2 we need to use pkg-config.

diff -ruN poppler-0.33.0.orig/CMakeLists.txt poppler-0.33.0/CMakeLists.txt
--- poppler-0.33.0.orig/CMakeLists.txt	2015-05-14 20:22:31.000000000 +0200
+++ poppler-0.33.0/CMakeLists.txt	2015-05-24 20:39:30.163709257 +0200
@@ -172,7 +172,7 @@
   set(USE_OPENJPEG1 ${LIBOPENJPEG_FOUND})
   set(WITH_OPENJPEG ${LIBOPENJPEG_FOUND})
 elseif(ENABLE_LIBOPENJPEG STREQUAL "openjpeg2")
-  find_package(LIBOPENJPEG2)
+  pkg_check_modules(LIBOPENJPEG2 libopenjp2)
   set(USE_OPENJPEG2 ${LIBOPENJPEG2_FOUND})
   set(WITH_OPENJPEG ${LIBOPENJPEG2_FOUND})
 endif()