gcc: fix c++ headers when same triplet cross compiling

When build platform and host platform differ, but have the same
triple, the code in nixpkgs will consider it a cross compilation,
but gcc won't. This will lead some derivations to look for c++
headers in the wrong place. To solve this always output the headers
in the non-cross location, like we do for the other gcc headers
already.

pkgs/development/compilers/gcc/common/configure-flags.nix

@@ -135,6 +135,8 @@ let
       # We pick "/" path to effectively avoid sysroot offset and make it work
       # as a native case.
       "--with-build-sysroot=/"
+      # Same with the stdlibc++ headers embedded in the gcc output
+      "--with-gxx-include-dir=${placeholder "out"}/include/c++/${version}/"
     ]
 
     # Basic configuration