[Bug preprocessor/31186] -I/usr/include not taken into account

[vincent-gcc at vinc17 dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=31186

--- Comment #6 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
OK, so I think there is a bug in the autotools, which use -I instead of
-isystem (unless this has changed).