Compatibility with webR

[lionel-]

• Disable the pthread code path
• Turn off visibility of symbols to avoid conflict with glue. Should also help people at google.

[gaborcsardi]

Thanks! Btw. if there is no bg thread, then I think the progress bars won't ever update. I am not entirely sure, but if this is the case, then we need an alternate thread-less implementation for the progress bar ticking.