Fix Autogen not detecting the MSYS Platform

[MeFisto94]

For some reason, autoconf and configure fail to guess the current platform, even though it should be working.

[MeFisto94]

Interesting find:
On my system it returns:

$ echo $OS
Windows_NT

$ uname -a
MINGW32_NT-6.2 HOSTNAME 1.0.19(0.48/3/2) 2016-07-13 17:45 i686 Msys

Let's see what happens to msys in appveyor. Maybe it correctly reports itself as MSYS_NT instead of WINDOWS_NT?

[MeFisto94]

From inside C:\MinGW\msys\1.0\bin\bash.exe we get a different uname -a:

bash-3.1$ uname -a
MSYS_NT-6.3 APPVYR-WIN 2.10.0(0.325/5/3) 2018-02-12 08:49 x86_64 Msys

This is because I used to run msys.bat, which sets up MSYSTEM and a few others (which are mostly for the terminal and hence not needed), however we have to run bash.exe directly, because the batch script chokes all the parameters (as it's intended to be used as an interactive shell)

[MeFisto94]

Just for reference: There is a mismatch between x86_64 and i686 there, however we specified MINGW32 as system, hence it's unclear why there still is an x86_64 and why that leads to linker errors.