Fix crash with ns_alert_dialog on Mac/Linux

Not sure why this wasn't crashing on windows... possibly which thread
is main thread is different on windows, so there the alert already runs
in the main thread? If so, then this probably needs #ifdef _WIN32
guards. Otherwise maybe it was just lucky that windows is more tolerant?
Throttle FLTK idle function to not use 100% CPU

Defining an idle function makes it run as frequently as possible. This
was causing the worm browser to chew up 100% CPU (on a mac at least).
This seems to be less of a problem on windows? Not sure why. If the
idle functions need to run more often that 20 fps, that can be changed
of course. Or is there a reason they need to constantly loop?
Responsiveness of menus etc. is unaffected even at 20 fps.
Add better description for --enable-browser
Don't double-build object files

Convince automake to not make a separate set of object files for all
the shared sources when building the worm browser.
Build ns_image_server_barcodes with automake.
Only wait for input before exit on Windows
Fix barcode encoding with RGB images