If you don't want to overwrite the system copy of v8 in /usr/lib/libv8.so,
you can use the --with-v8=<path> option to have php-v8js use its own
copy of libv8.
Note that threads are implemented using std::thread which is only available in C++0x. The relevant compile flags have been added but compiler support has not been tested and is therefore not guaranteed.