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

--- poppler-0.62.0/CMakeLists.txt
+++ poppler-0.62.0/CMakeLists.txt
@@ -193,7 +193,7 @@
 endif()
 set(WITH_OPENJPEG FALSE)
 if(ENABLE_LIBOPENJPEG STREQUAL "openjpeg2")
-  find_package(LIBOPENJPEG2)
+  pkg_check_modules(LIBOPENJPEG2 libopenjp2)
   set(WITH_OPENJPEG ${LIBOPENJPEG2_FOUND})
   if(NOT LIBOPENJPEG2_FOUND)
     message(FATAL_ERROR "Install libopenjpeg2 before trying to build poppler. You can also decide to use the internal unmaintained JPX decoder or none at all.")