In theory they shouldn't need anything outside of libsa, but disk.h and
bootstrap.h are currently required. Future work wil address this issue.
This fixes the build with MK_LOADER_ZFS=no. ZFS's Makefile.inc adds
these flags globally to CFLAGS when it should not. This masked the
problem because the tools/boot/universe.sh didn't build MK_LOADER_ZFS=no
as part of its regressions. Future work will also fix this.
Obtained from: CheriBSD
Fixes: 84bf2bbbec stand: constrain zlib/gzip CFLAGS better
Sponsored by: DARPA
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35860