Add Msys2 building appveor

[alex85k]

Following #224 and #226

Here is the build on Windows with latest MinGW compiler provided by MSYS2 project. The dependencies are also provied and downloaded as binary pacakages (may be updating, versions are not fixed).

I have also found a source of minor problem with not-finding json ruby gem - gem executable was called gem.cmd and was not found with FindGem.cmake.
With current version of MSYS2 packages all the tests should pass.

[codecov-io]

Codecov Report

Merging #227 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
+ Coverage   91.45%   91.48%   +0.02%     
==========================================
  Files         258      258              
  Lines       18006    18006              
==========================================
+ Hits        16468    16472       +4     
+ Misses       1538     1534       -4

Flag	Coverage Δ
#data_tests	79.48% <ø> (+0.08%)	⬆️
#examples	59.53% <ø> (+0.37%)	⬆️
#unit_tests	88.63% <ø> (+0.11%)	⬆️

Impacted Files	Coverage Δ
include/osmium/thread/queue.hpp	100% <0%> (+12.9%)	⬆️

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[joto]

I have merged this, thank you!

There are some warnings in the appveyor log about winsock2.h and windows.h. Do we need to do something there?

[alex85k]

The best thing to do about that warnings is to eliminate winsock headers from pbf reader. The only need for them is htonl function here:

libosmium/include/osmium/io/detail/pbf_output_format.hpp

Line 188 in 381cf92

const uint32_t sz = htonl(static_cast_with_assert<uint32_t>(blob_header_data.size()));

Will make PR with this simplification (now that we have protozero with similar functionality)