Fix makefs bootstrap on macOS after D25563

The macOS assert.h header does not define static_assert when compiling in
C99 mode. To fix this compile with -std=c11.

Reviewed By:	emaste
Differential Revision: https://reviews.freebsd.org/D25928

sbin/newfs_msdos/Makefile

@@ -9,5 +9,6 @@ SRCS=	newfs_msdos.c mkfs_msdos.c
 .if ${MACHINE_CPUARCH} == "arm"
 WARNS?= 3
 .endif
+CSTD=	c11
 
 .include <bsd.prog.mk>

usr.sbin/makefs/Makefile

@@ -17,6 +17,7 @@ SRCS=	cd9660.c \
 MAN=	makefs.8
 
 WARNS?=	2
+CSTD=	c11
 
 .include "${SRCDIR}/cd9660/Makefile.inc"
 .include "${SRCDIR}/ffs/Makefile.inc"