- Add Mono builds for Android.
- Remove now unused `MONO*_PREFIX` env variables (replaced by `mono_prefix`
command line option).
- Update iOS to SDK 12.4 and darwin17 for proper ARKit support.
- Drop 32-bit support for iOS (armv7, x86).
- Drop unused mandatory -t argument in build.sh.
- Remove infinite retries for OSX and iOS, builds now seem to
succeed right away and it prevents exiting the script.
- Factor out iOS SDK version.
- Make build order consistent in build.sh and build-release.sh
- Removing trailing whitespace.