Fix various errors caused by invalid const usage.

The previous commit was supposed to fix the issues caused by this.
If you compiled the code with -O2 or higher the tag lines for namespaces
contained the previous line as the command field, but this caused other
problems for other people too, see:
http://sourceforge.net/p/ctags/bugs/312/

Alternatively, you could define FILE_WRITE in every file where
this bug manifests too, but this patch makes it easier to fix these kinds
of errors.
The patch was inspired by the homebrew's ctags recipe.