Initialize v8js_globals manually only if ZTS is enabled.

If ZTS is disabled, the v8js_globals instance is declared right
in the BSS and hence automatically initialized by C++ compiler.
Most of the variables are just zeroed.

If ZTS is enabled however, v8js_globals just points to a freshly
allocated, unitialized piece of memory, hence we need to
initialize all fields on our own.  Likewise on shutdown we have to
run the destructors manually.
Merge pull request #69 from stesie/fix-global-zts-init

Initialize v8js_globals manually only if ZTS is enabled.