Porting MinGW multithreading bugfix to makefile.mingw
Porting MinGW multithreading bugfix to makefile.mingw

Original fix:
https://github.com/bitcoin/bitcoin/commit
/8864019f6d88b13d3442843d9e6ebeb8dd938831

reddink/reddcoin,Cannacoin-Project/Cannacoin,reddink/reddcoin,joroob/reddcoin,bmp02050/ReddcoinUpdates,joroob/reddcoin,joroob/reddcoin,reddink/reddcoin,ahmedbodi/poscoin,coinkeeper/2015-06-22_19-10_cannacoin,coinkeeper/2015-06-22_18-46_reddcoin,bmp02050/ReddcoinUpdates,bmp02050/ReddcoinUpdates,coinkeeper/2015-06-22_18-46_reddcoin,coinkeeper/2015-06-22_19-10_cannacoin,bmp02050/ReddcoinUpdates,reddcoin-project/reddcoin,ahmedbodi/poscoin,reddcoin-project/reddcoin,ahmedbodi/poscoin,Cannacoin-Project/Cannacoin,reddcoin-project/reddcoin,Cannacoin-Project/Cannacoin,Cannacoin-Project/Cannacoin,reddink/reddcoin,reddink/reddcoin,ahmedbodi/poscoin,joroob/reddcoin,reddcoin-project/reddcoin,coinkeeper/2015-06-22_18-46_reddcoin,coinkeeper/2015-06-22_18-46_reddcoin,coinkeeper/2015-06-22_18-46_reddcoin,coinkeeper/2015-06-22_19-10_cannacoin,coinkeeper/2015-06-22_19-10_cannacoin,reddcoin-project/reddcoin,Cannacoin-Project/Cannacoin,reddcoin-project/reddcoin,ahmedbodi/poscoin,joroob/reddcoin,bmp02050/ReddcoinUpdates,coinkeeper/2015-06-22_19-10_cannacoin