gitui-git: fix license dir and pkgver(); disable LTO

[eclairevoyant]

• License should be installed to /usr/share/licenses/$pkgname/ as per guidelines
• pkgver() needs git describe --long --tags to generate correct version, due to recent unannotated release tags (without --tags it reports as 0.19.0.r418.gc1e20958)
• Needs LTO off to prevent following error during clean chroot build: https://gist.githubusercontent.com/eclairevoyant/5bceeb72b7d16f8327202229b0954e4c/raw/1b8828c0870e4a8cd74d2b9e9ba69236e47f90c7/error%2520log