Add support for a system-wide shared libraries

[blshkv]

First of all, thank you for fixing the issue #94 , this is a good progress.
As the way of improvement, I would like to suggest implementing support of system-wide dependencies as well. For example, there are few tools which require capstone, libdwarf and all of them want to compile their own copy and link to it statically and there are a lot obvious disadvantages of such approach.

There is a way to verify if a system-wide library matches retdec requirements using pkg-config or cmake files and it should be possible to compile against them instead of compiling a static copy.
The tools which do not match can be still compiled locally (optionally).

The full list of the external tools is getting downloaded to the following location: external/src

[PeterMatula]

Yes, this could be done. We might play work on this in the future, but it is not priority at the moment. If anyone would like to help in the meantime, please go on.

[blshkv]

I have tried to unbundle some packages but hit the very old llvm (3.0) compiler quicky. It will take way too many changes to upgrade to v5 so I gave up at this point

[blshkv]

@PeterMatula
This is a good start:
https://github.com/avast/retdec/blob/master/deps/openssl/CMakeLists.txt
RETDEC_FORCE_OPENSSL_BUILD

Please implement similar variables for the remaining deps packages

[blshkv]

This is another short stopper:
https://github.com/avast/retdec/blob/master/src/bin2llvmir/utils/llvm.cpp#L382-L402
I'm unable to compile with system llvm just because of that part.

Possible solution:
https://stackoverflow.com/questions/16296480/iterate-over-all-structs-in-a-module/16297885