[Bug preprocessor/122411] Empty path in COMPILER_PATH leads to -isystem ./include

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122411

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is some documentation in the specs part though:
```
@item %I
Substitute any of @option{-iprefix} (made from @env{GCC_EXEC_PREFIX}),
@option{-isysroot} (made from @env{TARGET_SYSTEM_ROOT}),
@option{-isystem} (made from @env{COMPILER_PATH} and @option{-B} options)
and @option{-imultilib} as necessary.
```