Compile error: fatal error: sys/cdefs.h: No such file or directory [AND] fatal error: bits/c++config.h: No such file or directory

I was trying to compile it on Linux Ubuntu 16.04 and I had that error:

```
scons platform=linux generate_bindings=yes
scons: Reading SConscript files ...
scons: done reading SConscript files.
scons: Building targets ...
g++ -o src/core/String.o -c -fPIC -g -std=c++14 -Wwrite-strings -Og -m32 -I. -Igodot_headers -Iinclude -Iinclude/gen -Iinclude/core src/core/String.cpp
In file included from /usr/include/stdint.h:25:0,
from /usr/lib/gcc/x86_64-linux-gnu/5/include/stdint.h:9,
from godot_headers/gdnative/string.h:38,
from include/core/String.hpp:4,
from src/core/String.cpp:1:
/usr/include/features.h:367:25: fatal error: sys/cdefs.h: No such file or directory
compilation terminated.
scons: *** [src/core/String.o] Error 1
scons: building terminated because of errors.
```
I found out that I needed that lib: **libc6-dev-i386**
than I had this next error

`/usr/include/c++/5/cmath:41:28: fatal error: bits/c++config.h: No such file or directory`

Which was fixed installing those libs
- gcc-multilib
- g++-multilib

And it finally compiled

Have I forgotten to read it somewhere or it wasn't documented?
I've opened this issue because I thought that it might be useful to other people.