bug#30756: GCC >= 6 '-isystem' and C_INCLUDE_PATH behavior changed, breaking

[Ludovic Courtès]

Pushed as 2073b55e6b964cb8ca15e8c74cb32dac00f05f0d!

I’ve been able to build Python, CMake things, and I think Meson things,
so it’s looking good.

Next we should look at CROSS_CPATH.

Ludo’.