[Releasing] Updating the mingw version for 5.4 (CI/packaging)

[Koehne Kai]

Hi,

I'd like to suggest updating the MinGW version we build binaries for once more for Qt 5.4. The suggested toolchain would be mingw-builds i686-4.9.1-release-posix-dwarf-rt_v3-rev1 [1].

The toolchain includes:
*  gcc 4.9.1
* mingw-w64 headers from version v3.2.0 (4.8.2 contained ~ 3.0.0)
* GNU gdb 7.8  (4.8.2 contained 7.7)

Pros:
* Latest gcc 4.9. That obviously include some compiler fixes, like one for QTBUG-39646 .
* I've been testing it extensively in the last weeks, and didn't run into issues.
* There haven't been any source fixes necessariy (unlike in previous updates). 

Cons:
* i686-4.8.2-release-posix-dwarf-rt_v3-rev3 has been working fine. In addition to Qt 5.2 we also released 4.8.6 with it.
* It's pretty late in the process. Sorry for that :(


So, to sum it up: Switching to the new toolchain seems to be a low risk thing, and shipping the latest gcc is certainly a nice thing for our users. But then again, there are no smoking guns that would force us to update, either ...

[1]: http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/4.9.1/threads-posix/dwarf/i686-4.9.1-release-posix-dwarf-rt_v3-rev1.7z/download