Patch from Bastian Blank:

The updated patch adds a config option to explicitely enable 64 bit
    arithmetic.

    Also it removes the arith prototype from libbb.h as it is not used
    outside of ash.

    Bastian

this patch has been slightly modified by Erik for cleanliness.

include/libbb.h

@@ -289,8 +289,6 @@ char *concat_path_file(const char *path, const char *filename);
 char *concat_subpath_file(const char *path, const char *filename);
 char *last_char_is(const char *s, int c);
 
-extern long arith (const char *startbuf, int *errcode);
-
 int read_package_field(const char *package_buffer, char **field_name, char **field_value);
 //#warning yuk!
 char *fgets_str(FILE *file, const char *terminating_string);