Merge branch 'feature/support-mingw' into 'master'

Oliver Sanderrequested to merge
cherry-pick-bb8c8ad8 into releases/2.6

Feature/support mingw

See merge request !57 (merged)

(cherry picked from commit bb8c8ad8)

1dd861f6 [cmake] add check for rpc/rpc.h 83a54eda [mingw] make file IO work with mingw e3e4eeb7 [cmake] protect the RPC define to avoid problems of multily defined tests

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