diff --git a/cmake/Modules/OpmFind.cmake b/cmake/Modules/OpmFind.cmake
index 091f0500..77aa9969 100644
--- a/cmake/Modules/OpmFind.cmake
+++ b/cmake/Modules/OpmFind.cmake
@@ -42,6 +42,7 @@ set (_opm_proj_vars
   INCLUDE_DIRS
   LIBRARY_DIRS
   CONFIG_VARS
+  CONFIG_IMPL_VARS
   )
 
 # ensure that they are at least the empty list after we're done
diff --git a/cmake/Modules/OpmLibMain.cmake b/cmake/Modules/OpmLibMain.cmake
index ad2e4d5d..ca68ccb2 100644
--- a/cmake/Modules/OpmLibMain.cmake
+++ b/cmake/Modules/OpmLibMain.cmake
@@ -129,11 +129,13 @@ include (ConfigVars)
 list (APPEND ${project}_CONFIG_VARS ${${project}_CONFIG_VAR})
 
 # write configuration variables to this file. note that it is a temporary.
+# _CONFIG_IMPL_VARS are defines that are only written to config.h internal
+# to this project; they are not exported to any installed files.
 message (STATUS "Writing config file \"${PROJECT_BINARY_DIR}/config.h\"...")
 set (CONFIG_H "${PROJECT_BINARY_DIR}/config.h.tmp")
 configure_vars (
 	FILE  CXX  ${CONFIG_H}
-	WRITE ${${project}_CONFIG_VARS}
+	WRITE ${${project}_CONFIG_VARS} ${${project}_CONFIG_IMPL_VARS}
 	)
 
 # call this hook to let it setup necessary conditions for Fortran support