Build with Gomobile

[Nutomic]

I have started to try and make our build work with gomobile. If we can get this to work, it would give us a bunch of advantages over the existing build process:

• no more hacks needed to make DNS work
• we can get rid of all the custom bash scripts to build Syncthing
• fixes issues Syncthing binary: Build with GOOS=linux crashes > v0.14.1 #706, Build Syncthing with cgo and GOOS=android #873, Syncthing log timestamps should respect the system's timezone setting #960
• we can eventually add sdcard support using gobind (allows calling Android Java APIs from Go)

There are still some problems with this. First of all, the build isn't working and throwing errors (output is here). But I have checked it with golang.org/x/mobile/example/bind/hello instead of Syncthing and it worked, so it seems to be a problem in Syncthing?

Also, gomobile doesn't seem to support building ARMv5 binaries, and creates only ARMv7 instead. I've created a question on Stackoverflow for that.

Setup instructions

Setup Go and Android build tools first. Then run:

go get golang.org/x/mobile/cmd/gomobile
go get golang.org/x/mobile/cmd/gobind
gomobile init -ndk /path/to/ndk
go get github.com/syncthing/syncthing/cmd/syncthing

Then just compile with ./gradlew assembleDebug.

ping @calmh @Zillode @AudriusButkevicius

[Nutomic]

Gomobile officially requires ARMv7: golang/go#22108 (comment). That means we will have to drop support for those devices after switching to Gomobile.