[PATCH] D52524: Add -Wpoison-system-directories warning

[Sam Clegg via Phabricator via cfe-commits]

sbc100 added a comment.

Would it make sense to have this apply to system library paths too?   i.e. `-L/usr/local/lib`.

We have a bespoke mechanism in emscripten right now that is a bit more flexible but I'd love to simple replace it with `-Wpoison-system-directories`.

The name "system directories" seems to imply that it should apply to both include and library directories.  It also looks like the gcc version applies to both `Warn for -I and -L options using system directories if cross compiling` (from https://github.com/Metrological/buildroot/blob/master/package/gcc/4.8.4/910-gcc-poison-system-directories.patch).


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  https://reviews.llvm.org/D52524/new/

https://reviews.llvm.org/D52524