Export whether module is installed with package configuration.

Variable ProjectName_INSTALLED can be used to query whether this
is an installed module or not.
E.g. this is necessary to adapt include paths for dune-uggrid.

cmake/modules/DuneMacros.cmake

@@ -821,6 +821,8 @@ macro(finalize_dune_project)
     # Generate a standard cmake package configuration file
     file(WRITE ${PROJECT_BINARY_DIR}/CMakeFiles/${ProjectName}-config.cmake.in
 "if(NOT ${ProjectName}_FOUND)
+# Whether this module is installed or not
+set(${ProjectName}_INSTALLED @MODULE_INSTALLED@)
 @PACKAGE_INIT@
 
 #report other information
@@ -866,6 +868,7 @@ endif()")
   # Set the location of the doc file source. Needed by custom package configuration
   # file section of dune-grid.
   set(DUNE_MODULE_SRC_DOCDIR "${PROJECT_SOURCE_DIR}/doc")
+  set(MODULE_INSTALLED ON)
 
   configure_package_config_file(${CONFIG_SOURCE_FILE}
     ${PROJECT_BINARY_DIR}/cmake/pkg/${ProjectName}-config.cmake
@@ -889,6 +892,7 @@ macro(set_and_check _var _file)
     message(FATAL_ERROR \"File or directory \${_file} referenced by variable \${_var} does not exist !\")
   endif()
 endmacro()")
+  set(MODULE_INSTALLED OFF)
   configure_file(
     ${CONFIG_SOURCE_FILE}
     ${PROJECT_BINARY_DIR}/${ProjectName}-config.cmake @ONLY)