Add dependency back on libv8

therubyracer now delegates all lookup for its v8
compilation dependencies to the libv8 gem.

if you pass the --with-system-v8 flag to the libv8
gem on install, try to configure therubyracer upon
compile with the following options.

--with-v8-include, --with-v8-lib and --with-v8-dir

Otherwise, it will configure therubyracer to use
the vendored version of v8.