diff --git a/sys/conf/files b/sys/conf/files
--- a/sys/conf/files
+++ b/sys/conf/files
@@ -4100,34 +4100,34 @@
 libkern/timingsafe_bcmp.c	standard
 contrib/zlib/adler32.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/compress.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-cast-qual -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/crc32.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} ${NO_WSTRICT_PROTOTYPES}"
+	compile-with "${ZLIB_C}"
 contrib/zlib/deflate.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-cast-qual -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/inffast.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/inflate.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/inftrees.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/trees.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} ${NO_WSTRICT_PROTOTYPES}"
+	compile-with "${ZLIB_C}"
 contrib/zlib/uncompr.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} -Wno-cast-qual -Wno-deprecated-non-prototype"
+	compile-with "${ZLIB_C}"
 contrib/zlib/zutil.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib \
-	compile-with "${NORMAL_C} ${NO_WSTRICT_PROTOTYPES}"
+	compile-with "${ZLIB_C}"
 dev/zlib/zlib_mod.c		optional crypto | geom_uzip | \
 	mxge | ddb_ctf | gzio | zfs | zlib
 dev/zlib/zcalloc.c		optional crypto | geom_uzip | \
diff --git a/sys/conf/kern.pre.mk b/sys/conf/kern.pre.mk
--- a/sys/conf/kern.pre.mk
+++ b/sys/conf/kern.pre.mk
@@ -278,6 +278,11 @@
 DTRACE_C=	${CC} -c ${DTRACE_CFLAGS}	${WERROR} ${.IMPSRC}
 DTRACE_S=	${CC} -c ${DTRACE_ASM_CFLAGS}	${WERROR} ${.IMPSRC}
 
+# zlib code supports systems that are quite old, but will fix this issue once C2x gets radified.
+# see https://github.com/madler/zlib/issues/633 for details
+ZLIB_CFLAGS=	-Wno-cast-qual ${NO_WDEPRECATED_NON_PROTOTYPE} -Wno-strict-prototypes
+ZLIB_C=		${CC} -c ${CFLAGS} ${WERROR} ${ZLIB_CFLAGS} ${.IMPSRC}
+
 # Special flags for managing the compat compiles for DTrace/FBT
 FBT_CFLAGS=	-DBUILDING_DTRACE -nostdinc -I$S/cddl/dev/fbt/${MACHINE_CPUARCH} -I$S/cddl/dev/fbt ${CDDL_CFLAGS} -I$S/cddl/compat/opensolaris -I$S/cddl/contrib/opensolaris/uts/common  
 .if ${MACHINE_CPUARCH} == "amd64" || ${MACHINE_CPUARCH} == "i386"