Firebird doesn't support MSVC 2015

[David Ostrovsky]

So, after fixing number of MSVC 14.0 related bugs in FB
module: [1], I filed this bug upstream: [2] and got there
this comment from Egor Pugin:

"I tried to compile fb with MSVC2015. 
After fixing macro issues, you'll see more errors and more serious. The
list of errors is almost endless. (Of course no, but it's huge.) 
There will be errors that can change the runtime behaviour. So, it's
better to coordinate your efforts with FB devs and not do all
yourself." 

I see two options:

* Declare FB only supported on platforms, where it can be built with
current compiler toolchain that is used and decided by this project. 
* Cancel MSVC 14.0 upgrade for now and wait until it's supported by FB
project

* [1] https://gerrit.libreoffice.org/22757
* [2] http://tracker.firebirdsql.org/browse/CORE-5120