wg: compile on gnu99

We don't actually use any C11 features, so we can at least compile with
ancient gcc.

Reported-by: Aaron M. D. Jones <aaronmdjones@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

src/Makefile

@@ -38,7 +38,7 @@ endif
 PLATFORM ?= $(shell uname -s | tr '[:upper:]' '[:lower:]')
 
 CFLAGS ?= -O3
-CFLAGS += -std=gnu11 -D_GNU_SOURCE
+CFLAGS += -std=gnu99 -D_GNU_SOURCE
 CFLAGS += -Wall -Wextra
 CFLAGS += -MMD -MP
 CFLAGS += -DRUNSTATEDIR="\"$(RUNSTATEDIR)\""