[pkg] update build script for openvpn

Makefile

bitmask.pro

branding/installer/osx/se.leap.bitmask-helper.plist

branding/installer/post-install.py

branding/thirdparty/openvpn/build.mk

branding/thirdparty/openvpn/build_openvpn.sh

gui/backend.go

gui/build.sh

gui/main.cpp

installer/bitmask-installer.pro

installer/config/config.xml

installer/packages/riseupvpn/data/.gitignore

installer/packages/riseupvpn/data/post-install.py

...ller/packages/riseupvpn/data/se.leap.bitmask-helper.plist

pkg/helper/args.go

pkg/helper/darwin.go

pkg/helper/linux.go

pkg/helper/windows.go