Support linking to LLVM 3.5.

When using LLVM 3.5 one now has to include the --system-libs option when
linking. This option includes flags for linking to zlib and other libraries
required by LLVM. Previously this would be handled by other flags such as
--ldflags. From what I can gather the order is also important: system-libs has
to come after ldflags.

To implement this I've added the config option llvm_ldflags and moved generating
these flags to the configure script (instead of rakelib/blueprint.rb). This
change also fixed a small Windows issue that I noticed: the blueprint code would
call #sub on the ldlags Array, but that method only exists on the String class.

Worth noting, Julia seems to have had the same problem with LLVM 3.5:
https://github.com/JuliaLang/julia/pull/7696.