+# Expose the public headers to the implementation. We use `-isystem` for
+# purpose for two reasons:
+#
+# 1. It only does `<...>` not `"...."` lookup. We are making a libc,
+# which is a system library, so all public headers should be
+# accessible via `<...>` and never also need `"..."`. `-isystem` main
+# purpose is to only effect `<...>` lookup.
+#
+# 2. The `-I` for private headers added for specific C files below
+# should come earlier in the search path, so they can "override"
+# and/or `#include_next` the public headers. `-isystem` (like
+# `-idirafter`) comes later in the search path than `-I`.
+CFLAGS += -isystem "$(SYSROOT_INC)"