build: Use Go's default GOPATH if it's valid
build: Use Go's default GOPATH if it's valid

As of Go 1.8 it's valid to not have a GOPATH set. We ask the Go tool
what the GOPATH seems to be, and if we find ourselves (or a valid copy
of ourselves...) in that location we do not fiddle with the GOPATH.

wweich/syncthing,jryans/syncthing,syncthing/syncthing,rumpelsepp/syncthing,syncthing/syncthing,syncthing/syncthing,lkwg82/syncthing,uok/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,imsodin/syncthing,uok/syncthing,jryans/syncthing,imsodin/syncthing,jryans/syncthing,syncthing/syncthing,xduugu/syncthing,lkwg82/syncthing,syncthing/syncthing,lkwg82/syncthing,wweich/syncthing,xduugu/syncthing,rumpelsepp/syncthing,xduugu/syncthing,AudriusButkevicius/syncthing,jryans/syncthing,xduugu/syncthing,rumpelsepp/syncthing,wweich/syncthing,wweich/syncthing,syncthing/syncthing,imsodin/syncthing,imsodin/syncthing,AudriusButkevicius/syncthing,uok/syncthing,rumpelsepp/syncthing,lkwg82/syncthing,AudriusButkevicius/syncthing,uok/syncthing