"comply" with ISO C11

Except for the bunch of macros prefixed with
double underscores, but let's take care of
that some time later (read: never).

CMakeLists.txt

@@ -3,7 +3,7 @@
 cmake_minimum_required(VERSION 3.14.0)
 project(neo VERSION 0.1.0 LANGUAGES C)
 set(CMAKE_C_STANDARD 11)
-set(CMAKE_C_EXTENSIONS ON)
+set(CMAKE_C_EXTENSIONS OFF)
 
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME)
     set_property(GLOBAL PROPERTY USE_FOLDERS ON)

include/neo/_stddef.h

@@ -21,6 +21,10 @@ extern "C" {
 #	define offsetof(type, member) __builtin_offsetof(type, member)
 #endif
 
+#ifndef typeof
+#	define typeof(expr) __typeof(expr)
+#endif
+
 /** Get the absolute (non negative) value of an integer */
 #define nabs(n) ({								\
 	/* n is an expression, not a variable, evaluate it only once */		\