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

[redi at gcc dot gnu.org]

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

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
They should probably be using GCC_EXEC_PREFIX for that, which doesn't affect
the include search paths. But it looks like that's not something you can
change.