Actually respect the "--with-chisel" option

It turns out that you need to modify the build.scala in order to make
SBT actually use the local versions of Chisel.  The current autoconf
stuff wasn't doing that, which means that despite the fact that Chisel
was being built it wasn't actually being used.

This is still sub-optimal: it globally installs the Chisel version
you're using for Sodor.  I know nothing of SBT, so I'm just not going
to touch that... :).
Find the fesvr headers in a standard way

Originally (in other words, yesterday), sodor found the RISC-V tools
(fesvr was the only important one) by looking for them only in PREFIX,
which is a very non-standard way to do things.  Eric's recent patch
made them look in $RISCV, which while being the way that the rest of
the RISC-V tools do it is still fairly non-standard.  Both of these
approaches had the problem of failing with a cryptic error message at
compile time as opposed to failing when "./configure" was run.  This
patch changes this behavior to be more in line with what autoconf
normally does while maintaining backwards compatibility with both
methods.

Firstly, a new "--with-riscv" option has been added to "./configure".
If this option is passed then the RISC-V tools will be searched for in
the given directory.  If "--with-riscv" is NOT passed on the command
line, then configure will check if the RISCV environment variable is
set and if it is check for the RISC-V tools there.  If both of these
options are not set, then configure will search for the RISC-V tools
in PREFIX, which is always set (it defaults to "/usr/local").

Additionally, "./configure" now actually checks for the presence of
"fesvr/htif_pthread.h" when it is run and provides an error message if
that file does not exist.