Refactor layout of pxd files

capnp/helpers/checkCompiler.h

@@ -0,0 +1,11 @@
+#ifdef __GNUC__
+  #if __clang__
+    #if __cplusplus >= 201103L && !__has_include(<initializer_list>)
+      #warning "Your compiler supports C++11 but your C++ standard library does not.  If your system has libc++ installed (as should be the case on e.g. Mac OSX), try adding -stdlib=libc++ to your CFLAGS (ignore the other warning that says to use CXXFLAGS)."
+    #endif
+  #endif
+#endif
+
+#include "capnp/dynamic.h"
+
+static_assert(CAPNP_VERSION >= 3000, "Version of Cap'n Proto C++ Library is too old. Please upgrade to a version >= 0.3 and then re-install this python library");