Your bug report

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

which was filed against the guix package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 30756@debbugs.gnu.org.

-- 
30756: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=30756
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