As GCC sets `-D_FORTIFY_SOURCE=2` (maybe it's MSYS2 or `makepkg` well I
don't really care about which one it is), those `__*_chk()` functions
are now required when building GCC and its libraries. Unfortunately,
mingw-w64 does not provide these (it relies libssp to provide them), so
the build now fails with undefined references.
I tried adding `-fstack-protector` into `LDFLAGS` and adding `-lssp`
into `LIBS` when configuring GCC, but neither worked for me. The build
process errorred out when building either libiberty or libcpp.
Tampering with GCC builtin specs may be a solution, but I am not sure
whether libtool likes that. It ignores default libraries and always
attempts to link against everything at its own disposal.
I am not sure whether libssp is (rather, can be made) available when
building libiberty. Or this _FORTIFY_SOURCE thing should be disabled
when building GCC for mingw-w64.