build-sys: fix appending of CFLAGS and define __SANE_USERSPACE_TYPES__

build-sys: fix appending of CFLAGS and define __SANE_USERSPACE_TYPES__

It's pointless to call AC_SUBST more than once on the same variable. Because
of all the copypasta, we were mixing CLFAGS and LDFLAGS.

… and the assertion in previous commit was wrong. PPC64 is a special snowflake.

__SANE_USERSPACE_TYPES__ is needed on PPC64 to make __u64 be llu, instead of
lu. Considering that both lu and llu are 64 bits, there's nothing sane about
this, maybe the flag should be called __INSANE_USERSPACE_TYPES__ instead. Sane
or not, this makes ppc64 kernel headers behave consistent with other
architectures. With this flag, no warnings are emitted at -O0 level.