func-attr: add some extra handy function attrs

The attributes in question are:

- nonnull: specify whether a function argument cannot/may not be null
- returns_nonnull: specify whether a function will not return a null
  pointer (example: xmalloc can't return null, so it should be annotated as
  such). Only available from gcc 4.9 onwards.

Currently these attributes are only supported by gcc.

src/func_attr.h

@@ -40,9 +40,18 @@
   #elif defined(__INTEL_COMPILER)
     // intel only
   #else
+    #define GCC_VERSION \
+           (__GNUC__ * 10000 + \
+            __GNUC_MINOR__ * 100 + \
+            __GNUC_PATCHLEVEL__)
     // gcc only
     #define FUNC_ATTR_ALLOC_SIZE(x) __attribute__((alloc_size(x)))
     #define FUNC_ATTR_ALLOC_SIZE_PROD(x,y) __attribute__((alloc_size(x,y)))
+    #define FUNC_ATTR_NONNULL_ALL __attribute__((nonnull))
+    #define FUNC_ATTR_NONNULL_ARG(...) __attribute__((nonnull(__VA_ARGS__)))
+    #if GCC_VERSION >= 40900
+      #define FUNC_ATTR_NONNULL_RET __attribute__((returns_nonnull))
+    #endif
   #endif
 #endif
 
@@ -81,4 +90,16 @@
   #define FUNC_ATTR_ALWAYS_INLINE
 #endif
 
+#ifndef FUNC_ATTR_NONNULL_ALL
+  #define FUNC_ATTR_NONNULL_ALL
+#endif
+
+#ifndef FUNC_ATTR_NONNULL_ARG
+  #define FUNC_ATTR_NONNULL_ARG(...)
+#endif
+
+#ifndef FUNC_ATTR_NONNULL_RET
+  #define FUNC_ATTR_NONNULL_RET
+#endif
+
 #endif // NEOVIM_FUNC_ATTR_H