Export PREFIX to find config.h.cmake

[[Imported from SVN: r1721]]

cmake/pkg/dune-istl-config.cmake.in

@@ -7,6 +7,7 @@ get_filename_component(_prefix "${_dir}/../../.." ABSOLUTE)
 #include("${_prefix}/lib/cmake/@DUNE_MOD_NAME@-targets.cmake")
 
 #report other information
+set(@DUNE_MOD_NAME@_PREFIX "${_prefix}")
 set(@DUNE_MOD_NAME@_INCLUDE_DIRS "${_prefix}/include")
 set(@DUNE_MOD_NAME@_CXX_FLAGS "@CMAKE_CXX_FLAGS@")
 set(@DUNE_MOD_NAME@_CXX_FLAGS_DEBUG "@CMAKE_CXX_FLAGS_DEBUG@")