Add helper macros for wrapping annoying C types

We will wrap the file_handle struct in the next commit. The struct has
a flexible array member, which is not supported by C++. Compiler may
complain about using it when allocated on stack, even indirectly as a
member of a struct. I'm not sure if using this kind of types is even a
defined behavior…

src/act_helpers.h

+#pragma once
+
+// Annoying C type helpers.
+//
+// C headers sometimes contain struct ending with a flexible array
+// member, which is not supported in C++. An example of such a type is
+// file_handle from fcntl.h (man name_to_handle_at)
+//
+// Here are some helper macros helping to ensure if the C++
+// counterpart has members of the same type and offset.
+
+#include <type_traits>
+
+namespace act_helpers
+{
+
+template <typename T, typename M> M get_member_type(M T:: *);
+
+}
+
+#define ACTH_SAME_SIZE(cxxtype, ctype, extra_type) \
+  (sizeof(cxxtype) == sizeof(ctype) + sizeof(extra_type))
+#define ACTH_GET_TYPE_OF(mem) \
+  decltype(act_helpers::get_member_type(&mem))
+#define ACTH_SAME_OFFSET(cxxtype, cxxmem, ctype, cmem) \
+  (std::is_standard_layout<cxxtype>::value && \
+   std::is_standard_layout<ctype>::value && \
+   (offsetof(cxxtype, cxxmem) == offsetof(ctype, cmem)))
+#define ACTH_SAME_TYPE(cxxtype, cxxmem, ctype, cmem) \
+  std::is_same<ACTH_GET_TYPE_OF(cxxtype :: cxxmem), ACTH_GET_TYPE_OF(ctype :: cmem)>::value
+
+#define ACTH_ASSERT_SAME_SIZE(cxxtype, ctype, extra_type) \
+  static_assert(ACTH_SAME_SIZE(cxxtype, ctype, extra_type), \
+                "assumption that is broken: " #cxxtype " == " #ctype " + " # extra_type)
+#define ACTH_ASSERT_SAME_OFFSET(cxxtype, cxxmem, ctype, cmem) \
+  static_assert(ACTH_SAME_OFFSET(cxxtype, cxxmem, ctype, cmem), \
+                "assumption that is broken: " #cxxtype "::" #cxxmem " is at the same offset as " #ctype "::" #cmem)
+#define ACTH_ASSERT_SAME_TYPE(cxxtype, cxxmem, ctype, cmem) \
+  static_assert(ACTH_SAME_TYPE(cxxtype, cxxmem, ctype, cmem), \
+                "assumption that is broken: " #cxxtype "::" #cxxmem " has the same type as " #ctype "::" #cmem)
+#define ACTH_ASSERT_SAME_MEMBER(cxxtype, cxxmem, ctype, cmem) \
+  ACTH_ASSERT_SAME_TYPE(cxxtype, cxxmem, ctype, cmem); \
+  ACTH_ASSERT_SAME_OFFSET(cxxtype, cxxmem, ctype, cmem)