Restore old -std= flags

Getting rid of them fixed GA Ubuntu, but broke GA MacOS. Let's try a
different strategy.

Makefile

@@ -30,8 +30,8 @@ endif
 # Compile flags
 #
 
-CFLAGS   = -I.              -O3 -DNDEBUG -fPIC
-CXXFLAGS = -I. -I./examples -O3 -DNDEBUG -fPIC
+CFLAGS   = -I.              -O3 -DNDEBUG -std=c11   -fPIC
+CXXFLAGS = -I. -I./examples -O3 -DNDEBUG -std=c++11 -fPIC
 LDFLAGS  =
 
 # OS specific

mmap.h

@@ -1,5 +1,9 @@
 #pragma once
 
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <stddef.h>
 #include <stdint.h>
 #include <fcntl.h>