Debian moved to gcc-9. This gcc version now refuses -flto="Thin" Move from...

Debian moved to gcc-9. This gcc version now refuses -flto="Thin" Move from LLVM_ENABLE_LTO="Thin" to BOOTSTRAP_LLVM_ENABLE_LTO="Thin" to mitigate the issue (the argument should be passed only at the second stage of the build process) See https://bugs.llvm.org/show_bug.cgi?id=43193