Fix build flags to build properly with uClibc

Building with -std=c99 -D_DEFAULT_SOURCE hides the struct timespec and
nanosleep() definitions from <time.h> in uClibc. Work around that by
using -std=gnu99 instead.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/lua-periphery/Makefile
===================================================================
--- a/lua-periphery/Makefile
+++ b/lua-periphery/Makefile
@@ -16,7 +16,7 @@
 
 ###########################################################################
 
-CFLAGS += -std=c99 -pedantic -D_DEFAULT_SOURCE
+CFLAGS += -std=gnu99 -pedantic
 CFLAGS += -Wall -Wextra -Wno-unused-parameter $(DEBUG) -fPIC -I. $(LUA_CFLAGS)
 LDFLAGS += -shared