Default to using C99 standard.

mk/cc.mk

@@ -4,6 +4,11 @@
 
 CFLAGS?=	-O2 -pipe
 
+CSTD?=		c99
+_CSTD_SH=	if test -n "${CSTD}"; then echo "-std=${CSTD}"; else echo ""; fi
+_CSTD!=		${_CSTD_SH}
+CFLAGS+=	${_CSTD}$(shell _CSTD_SH)
+
 # GNU Make way of detecting gcc flags we can use
 check_gcc=$(shell if ${CC} $(1) -S -o /dev/null -xc /dev/null >/dev/null 2>&1; \
 	then echo "$(1)"; else echo "$(2)"; fi)
@@ -23,4 +28,3 @@ CFLAGS+=	-pedantic \
 		$(call check_gcc, -Wdeclaration-after-statement) \
 		$(call check_gcc, -Wsequence-point) \
 		$(call check_gcc, -Wextra) ${WEXTRA}
-