test for existance of country grid in cmake already

Given that the file potentially gets installed, it needs to be
present during build time already. Checking during the import
is too late.

CMakeLists.txt

@@ -97,6 +97,17 @@ endif()
 #-----------------------------------------------------------------------------
 
 if (BUILD_IMPORTER)
+   find_file(COUNTRY_GRID_FILE country_osm_grid.sql.gz
+             PATHS ${PROJECT_SOURCE_DIR}/data
+             NO_DEFAULT_PATH
+             DOC "Location of the country grid file."
+            )
+
+   if (NOT COUNTRY_GRID_FILE)
+       message(FATAL_ERROR "\nYou need to download the country_osm_grid first:\n"
+                           "    wget -O ${PROJECT_SOURCE_DIR}/data/country_osm_grid.sql.gz https://www.nominatim.org/data/country_grid.sql.gz")
+   endif()
+
    set(CUSTOMSCRIPTS
        check_import_finished.php
        country_languages.php
@@ -221,7 +232,7 @@ if (BUILD_IMPORTER)
     install(DIRECTORY lib-sql DESTINATION ${NOMINATIM_LIBDIR})
 
     install(FILES data/country_name.sql
-                  data/country_osm_grid.sql.gz
+                  ${COUNTRY_GRID_FILE}
                   data/words.sql
             DESTINATION ${NOMINATIM_DATADIR})
 endif()