[Makefile] Adding c99 standard

Makefile

@@ -21,6 +21,8 @@ CFLAGS += -m64 $(INCS) -pedantic -Wall -Wextra -Wshadow \
 
 LDFLAGS  += ${LIBS}
 
+CC += -std=c99
+
 ifeq ($(DEBUG), 0)
    CFLAGS  += -O3
 else