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

[p.e.c.melis at alumnus dot utwente.nl]

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

--- Comment #4 from Paul Melis <p.e.c.melis at alumnus dot utwente.nl> ---
I didn't configure it, but it's apparently for the XALT tool we have deployed,
which intercepts linking. See
https://xalt.readthedocs.io/en/latest/095_xalt_env_vars.html#xalt-s-environment-variables