Revert "Use INC_DIR for OPENJPEG_INCLUDE_DIRS (fixes uclouvain#1174)"

This reverts commit 6558637 which
breaks cross-compilation of poppler under buildroot (because of
DESTDIR usage).

cmake/OpenJPEGConfig.cmake.in

@@ -27,8 +27,12 @@ if(EXISTS ${SELF_DIR}/OpenJPEGTargets.cmake)
   # This is an install tree
   include(${SELF_DIR}/OpenJPEGTargets.cmake)
 
+  # We find a relative path from the PKG directory to header files.
+  set(PKG_DIR "@CMAKE_INSTALL_PREFIX@/@OPENJPEG_INSTALL_PACKAGE_DIR@")
   set(INC_DIR "@CMAKE_INSTALL_PREFIX@/@OPENJPEG_INSTALL_INCLUDE_DIR@")
-  get_filename_component(OPENJPEG_INCLUDE_DIRS "${INC_DIR}" ABSOLUTE)
+  file(RELATIVE_PATH PKG_TO_INC_RPATH "${PKG_DIR}" "${INC_DIR}")
+
+  get_filename_component(OPENJPEG_INCLUDE_DIRS "${SELF_DIR}/${PKG_TO_INC_RPATH}" ABSOLUTE)
 
 else()
   if(EXISTS ${SELF_DIR}/OpenJPEGExports.cmake)