Trying to compile uClibc

I'm brand new to Alpine. I want to compile libc myself, so I can use it in debugging. Maybe it's already compiled appropriately, but I would like the matching source.

- I've cloned aports
- in aports/main/libc0.9.32 I have done:
-- abuild -r builddeps
-- abuild clean unpack prepare
-- abuild build

Did I need to do anything more before attempting the build? In any case, I quickly get a whole bunch of messages complaining about va_list in stdio.h:

In file included from libpthread/nptl/sysdeps/unix/sysv/linux/gen_lowlevelrwlock.c:2:0:
./include/stdio.h:356:8: error: unknown type name '__gnuc_va_list'
__gnuc_va_list __arg);
^~~~~~~~~~~~~~

Any advice is most welcome.