kmod.mk includes bsd.sys.mk anyway so use CSTD instead of homegrown

reimplementation of the same. Note that this changes -std=c99
to -std=iso9899:1999 but those two are synonyms.

Approved by:	kib (mentor)
Reviewed by:	ru

sys/conf/kmod.mk

@@ -89,10 +89,9 @@ CFLAGS+=	-DKLD_MODULE
 .if ${CC} == "icc"
 NOSTDINC=	-X
 .else
-C_DIALECT=	-std=c99
+CSTD=		c99
 NOSTDINC=	-nostdinc
 .endif
-CFLAGS+=	${C_DIALECT}
 CFLAGS:=	${CFLAGS:N-I*} ${NOSTDINC} ${INCLMAGIC} ${CFLAGS:M-I*}
 .if defined(KERNBUILDDIR)
 CFLAGS+=	-DHAVE_KERNEL_OPTION_HEADERS -include ${KERNBUILDDIR}/opt_global.h