introduce constant for configuration directory

This replaces {data_dir}/settings throughout the code, so that
the configuration may be placed somewhere else in the directory
structure (e.g. in /etc).

cmake/tool.tmpl

@@ -13,4 +13,5 @@ exit(cli.nominatim(module_dir='@CMAKE_BINARY_DIR@/module',
                    phplib_dir='@CMAKE_SOURCE_DIR@/lib-php',
                    sqllib_dir='@CMAKE_SOURCE_DIR@/lib-sql',
                    data_dir='@CMAKE_SOURCE_DIR@',
+                   config_dir='@CMAKE_SOURCE_DIR@/settings',
                    phpcgi_path='@PHPCGI_BIN@'))