--- a/Makefile.inc
+++ b/Makefile.inc
@@ -101,8 +101,9 @@ OPTFLAGS	:= -O2 -g $(STACKPROT) --param=ssp-buffer-size=4
 WARNFLAGS	:= -Werror -Wall -Wextra -Wformat=2 $(WFORMATOVERFLOW) -Werror=implicit-int \
 		  -Werror=implicit-function-declaration -Werror=format-security \
 		  $(WNOCLOBBERED) -Werror=cast-qual $(ERROR_DISCARDED_QUALIFIERS)
-CPPFLAGS	:= -Wp,-D_FORTIFY_SOURCE=2
-CFLAGS		:= --std=gnu99 $(CFLAGS) $(OPTFLAGS) $(WARNFLAGS) -pipe \
+CPPFLAGS	?= -Wp,-D_FORTIFY_SOURCE=2
+CFLAGS		?= --std=gnu99 $(CFLAGS) $(OPTFLAGS) $(WARNFLAGS) -pipe
+CFLAGS		+= --std=gnu99 \
 		   -DBIN_DIR=\"$(bindir)\" -DLIB_STRING=\"${LIB}\" -DRUN_DIR=\"${RUN}\" \
 		   -MMD -MP
 BIN_CFLAGS	= -fPIE -DPIE