Update copyright years.
Update comment about `llvm-config' options.

In LLVM 3.5, `llvm-config --ldflags' does not contain a list of system
library linker options.  This is a good thing; it makes it easier for us
to generate good command lines for linking `clang_delta'.

Now one gets the system library directives with `... --system-libs'.

John Regehr already made the main change that was enabled by
`--system-libs': namely, we can easily include the system library
directives into `LLVMLIBS'.

We should also be able to go back to defining `clang_delta_LDFLAGS'
properly.  (See commit 6ce0a98 from June 7 2012.)  But I haven't made
this change yet.

This commit just updates the in-file comments about this issue.