Auto merge of #12573 - UK992:appveyor-fix, r=larsbergstrom
Auto merge of #12573 - UK992:appveyor-fix, r=larsbergstrom

Appveyor: Downgrade GCC to 5.4.0-1

Fix building on Appveyor.

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dsandeephegde/servo,KiChjang/servo,ConnorGBrewster/servo,upsuper/servo,fiji-flo/servo,emilio/servo,thiagopnts/servo,eddyb/servo,SimonSapin/servo,saneyuki/servo,cbrewster/servo,pyfisch/servo,sadmansk/servo,KiChjang/servo,dati91/servo,cbrewster/servo,saneyuki/servo,notriddle/servo,DominoTree/servo,fiji-flo/servo,saneyuki/servo,avadacatavra/servo,nnethercote/servo,rnestler/servo,eddyb/servo,CJ8664/servo,canaltinova/servo,eddyb/servo,nnethercote/servo,emilio/servo,fiji-flo/servo,pyfisch/servo,thiagopnts/servo,SimonSapin/servo,DominoTree/servo,CJ8664/servo,saneyuki/servo,canaltinova/servo,emilio/servo,dati91/servo,eddyb/servo,mbrubeck/servo,szeged/servo,fiji-flo/servo,anthgur/servo,upsuper/servo,mbrubeck/servo,notriddle/servo,nnethercote/servo,peterjoel/servo,notriddle/servo,dati91/servo,jimberlage/servo,peterjoel/servo,dati91/servo,CJ8664/servo,nnethercote/servo,larsbergstrom/servo,pyfisch/servo,dsandeephegde/servo,SimonSapin/servo,SimonSapin/servo,paulrouget/servo,notriddle/servo,nrc/servo,nnethercote/servo,eddyb/servo,splav/servo,mbrubeck/servo,rnestler/servo,larsbergstrom/servo,emilio/servo,ConnorGBrewster/servo,rnestler/servo,jimberlage/servo,peterjoel/servo,DominoTree/servo,thiagopnts/servo,canaltinova/servo,avadacatavra/servo,cbrewster/servo,upsuper/servo,nrc/servo,rnestler/servo,canaltinova/servo,SimonSapin/servo,saneyuki/servo,avadacatavra/servo,szeged/servo,upsuper/servo,CJ8664/servo,mbrubeck/servo,upsuper/servo,saneyuki/servo,anthgur/servo,dsandeephegde/servo,rnestler/servo,dsandeephegde/servo,pyfisch/servo,ConnorGBrewster/servo,notriddle/servo,jimberlage/servo,nrc/servo,mbrubeck/servo,notriddle/servo,upsuper/servo,larsbergstrom/servo,thiagopnts/servo,DominoTree/servo,larsbergstrom/servo,rnestler/servo,saneyuki/servo,eddyb/servo,DominoTree/servo,emilio/servo,avadacatavra/servo,sadmansk/servo,ConnorGBrewster/servo,notriddle/servo,emilio/servo,canaltinova/servo,nrc/servo,sadmansk/servo,mattnenterprise/servo,jimberlage/servo,szeged/servo,KiChjang/servo,splav/servo,sadmansk/servo,anthgur/servo,avadacatavra/servo,pyfisch/servo,CJ8664/servo,dsandeephegde/servo,sadmansk/servo,canaltinova/servo,avadacatavra/servo,emilio/servo,thiagopnts/servo,paulrouget/servo,peterjoel/servo,pyfisch/servo,peterjoel/servo,KiChjang/servo,DominoTree/servo,avadacatavra/servo,splav/servo,peterjoel/servo,jimberlage/servo,nrc/servo,dati91/servo,thiagopnts/servo,saneyuki/servo,dsandeephegde/servo,larsbergstrom/servo,mattnenterprise/servo,pyfisch/servo,peterjoel/servo,notriddle/servo,pyfisch/servo,eddyb/servo,upsuper/servo,mattnenterprise/servo,fiji-flo/servo,paulrouget/servo,splav/servo,paulrouget/servo,paulrouget/servo,dsandeephegde/servo,cbrewster/servo,dati91/servo,CJ8664/servo,emilio/servo,jimberlage/servo,pyfisch/servo,szeged/servo,KiChjang/servo,saneyuki/servo,mbrubeck/servo,SimonSapin/servo,szeged/servo,cbrewster/servo,saneyuki/servo,pyfisch/servo,fiji-flo/servo,szeged/servo,dsandeephegde/servo,upsuper/servo,szeged/servo,fiji-flo/servo,paulrouget/servo,CJ8664/servo,anthgur/servo,KiChjang/servo,splav/servo,nnethercote/servo,notriddle/servo,DominoTree/servo,ConnorGBrewster/servo,mattnenterprise/servo,mattnenterprise/servo,peterjoel/servo,rnestler/servo,anthgur/servo,paulrouget/servo,jimberlage/servo,splav/servo,larsbergstrom/servo,paulrouget/servo,jimberlage/servo,sadmansk/servo,thiagopnts/servo,peterjoel/servo,ConnorGBrewster/servo,splav/servo,paulrouget/servo,KiChjang/servo,avadacatavra/servo,DominoTree/servo,larsbergstrom/servo,ConnorGBrewster/servo,anthgur/servo,nrc/servo,szeged/servo,cbrewster/servo,anthgur/servo,rnestler/servo,anthgur/servo,notriddle/servo,SimonSapin/servo,szeged/servo,nnethercote/servo,thiagopnts/servo,mbrubeck/servo,dati91/servo,larsbergstrom/servo,mattnenterprise/servo,canaltinova/servo,nnethercote/servo,DominoTree/servo,peterjoel/servo,larsbergstrom/servo,szeged/servo,KiChjang/servo,dati91/servo,fiji-flo/servo,mattnenterprise/servo,splav/servo,splav/servo,splav/servo,nrc/servo,KiChjang/servo,canaltinova/servo,emilio/servo,nnethercote/servo,mbrubeck/servo,larsbergstrom/servo,mattnenterprise/servo,sadmansk/servo,nnethercote/servo,sadmansk/servo,emilio/servo,SimonSapin/servo,jimberlage/servo,ConnorGBrewster/servo,cbrewster/servo,cbrewster/servo,paulrouget/servo,DominoTree/servo,eddyb/servo,CJ8664/servo,KiChjang/servo