Ensure that json extension is enabled when running tests.

The `commonjs_modules.phpt` test requires `json_encode` in order to
complete successfully.
Support compiling v8js with v8 in a custom path.

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.
Merge pull request #35 from cscott/fixes

Makefile fixes.