No proper handling of external globals

I was wondering if retdec is able to work with complex executables dependent on other shared libraries. Seems like this scenario does not work:

```c
$ cat test.cc
extern int a;

int main()
{
return a;
}

$ cat a.c
int a = 1023;

// gcc -m32 test.cc -o test -L. -la
```

Comes out with wrong value for `a`:

```
$ cat test.c
//
// This file was generated by the Retargetable Decompiler
// Website: https://retdec.com
// Copyright (c) 2018 Retargetable Decompiler
//

#include

// --------------------- Global Variables ---------------------

int32_t g1 = 0;

// ------------------------ Functions -------------------------

// Address range: 0x804851b - 0x8048525
int main(int argc, char ** argv) {
// 0x804851b
return g1;
}

// --------------------- Meta-Information ---------------------

// Detected compiler/packer: gcc (5.5.0)
// Detected functions: 1
// Decompilation date: 2018-06-17 02:34:02
```

Start by reproducing the example from test.cc and a.c, then inspect the generated test.c and compare its handling of the external variable with the executable's result. Trace the decompiler's external-global and shared-library analysis entry points; done means the generated result correctly represents the external global rather than using an incorrect local value.

Written by the indexing model from the issue text.