Fix the issues with shared libraries

[abumq]

Currently when easylogging++ is used in shared libraries as well as the application linked to that shared libararies, it's pain to get it all working. We need to fix this as this is common design with large C++ applications.

NOTE: We will wait until #445 is finished so we can re-think design slightly

[aparajita]

I have static and shared libs working fine on macOS with Xcode. I'll push the projects sometime this week.

[abumq]

On XCode, it works for me too but when I used Qt Creator IDE or built it using cmake (on terminal) on thesame system it was crashing.

[aparajita]

Compile in Xcode and then copy the compile and link transcripts. There must be some configuration option Xcode is passing to clang/ld that the others are not.

[abumq]

Static lib is still problem but shared is working fine

See samples/stl/static-shared-lib

[abumq]

This is after #445 (issue with static)

[abumq]

It looks all good to me now. It's matter of compiling it right.