If the libcxx headers already have been installed, adding
-I$LIBCXX/include to the include path breaks some C standard
headers. The libcxx installation provides e.g. stdlib.h, which
does an #include_next <stdlib.h>. If the include path includes
both $LIBCXX/include and the libcxx headers installed to the
target sysroot, the #include_next will pick up the same header
from the next include directory. And due to include guards,
the second inclusion of the libcxx version of the header does
nothing at all, instead of doing yet another #include_next for
picking up the real version of the header.
By adding -nostdinc++, we avoid the possibly installed version
of these headers.
This fixes rebuilding libunwind after libcxx has been built
and installed.
Martin Storsjö