FreeBSD: decouple ZFS_DEBUG from kernel debug settings

Reviewed-by: Martelli Nikola @martellini
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #11213

module/Makefile.bsd

@@ -39,7 +39,13 @@ CFLAGS+= -DHAVE_AVX2 -DHAVE_AVX -D__x86_64 -DHAVE_SSE2 -DHAVE_AVX512F -DHAVE_SSS
 .endif
 
 .if defined(WITH_DEBUG) && ${WITH_DEBUG} == "true"
-CFLAGS+= -DINVARIANTS -DWITNESS -g -O0 -DZFS_DEBUG -DOPENSOLARIS_WITNESS
+CFLAGS+= -DZFS_DEBUG -g
+.if defined(WITH_INVARIANTS) && ${WITH_INVARIANTS} == "true"
+ CFLAGS+= -DINVARIANTS -DWITNESS -DOPENSOLARIS_WITNESS
+.endif
+.if defined(WITH_O0) && ${WITH_O0} == "true"
+ CFLAGS+= -O0
+.endif
 .else
 CFLAGS += -DNDEBUG
 .endif