[Bug preprocessor/63670] Ending C_INCLUDE_PATH with a trailing colon broken

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63670

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think this is the correct behaviour, and this is basically a dup of PR30439