Change contributed fix from gnu99 to c99

Makefile

@@ -5,11 +5,11 @@ B64_OBJS?=src/b64/cencode.o
 FORMAT_OBJS?=src/formats/json.o src/formats/raw.o src/formats/common.o src/formats/custom-type.o
 HTTP_PARSER_OBJS?=src/http-parser/http_parser.o
 
-CFLAGS ?= -Wall -Wextra -Isrc -Isrc/jansson/src -Isrc/http-parser -MD
+CFLAGS ?= -std=c99 -Wall -Wextra -Isrc -Isrc/jansson/src -Isrc/http-parser -MD
 LDFLAGS ?= -levent -pthread
 
 # Pass preprocessor macros to the compile invocation
-CFLAGS += $(CPPFLAGS) -std=gnu99
+CFLAGS += $(CPPFLAGS)
 
 # check for MessagePack
 MSGPACK_LIB=$(shell ls /usr/lib/libmsgpack.so 2>/dev/null)