Drop -flto from non-cross-language-lto C++ demo builds

It appears somehow the LLVM IR is slipping into the Rust library,
causing Ubunto link to fail with the following error:

/usr/bin/ld: error: LLVM gold plugin has failed to create LTO module: Invalid record
clang: error: linker command failed with exit code 1 (use -v to see invocation)