Root cause chain discovered and fixed:
1. GCC built-in stddef.h shadowed by relibc's _STDDEF_H guard
→ fix_types.h with guarded typedefs for 15+ sys types
2. gnulib configure bakes raw typedefs into GL_CFLAG_GNULIB_WARNINGS
→ strip them from Makefiles after configure
3. __fseterr/__freadahead don't exist in relibc
→ compile C stubs and inject into link via Makefile patch
The recipe pattern is documented and reusable for other gnulib packages
(ninja-build, diffutils, etc.).
Also: bootloader recipe needs RUSTFLAGS=-Zunstable-options for
custom target support after redoxer toolchain restore.
vasilito
8b0cabaa47