Enforce -std=gnu11, which enables typeof() in GCC 4.8

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/Makefile.am

@@ -1,7 +1,7 @@
 lib_LTLIBRARIES     = libuev.la
 libuev_la_SOURCES   = uev.c io.c timer.c signal.c cron.c
 libuev_la_CPPFLAGS  = -D_GNU_SOURCE -D_TIME_BITS=64
-libuev_la_CFLAGS    = -W -Wall -Wextra
+libuev_la_CFLAGS    = -W -Wall -Wextra -std=gnu11
 libuev_la_LDFLAGS   = $(AM_LDFLAGS) -version-info 2:2:0
 
 noinst_PROGRAMS     = bench