Add build-time libffi linking workaround for #12.
Add build-time libffi linking workaround for #12.

Setting `LLVM_SYS_*_FFI_WORKAROUND` at build time will force linking against
libffi, which is required if LLVM is built with support for libffi but is not
reported as a required system library by `llvm-config`.

tari/llvm-sys.rs,tari/llvm-sys.rs,tari/llvm-sys.rs