Use bitfield for node flags.
Added t4147 - initial test for ^o flag to skip commands.

If the ^o flag is present, all outputs are moved to .tup/tmp before running instead of unlinking. After the command completes, we can check them to see if they are equal. If they are, we can skip future commands that depend on those files (unless some other input has changed).
Added t4148 - check symlinks correctly with ^o flag.
Don't show skipped command banners.

If there are a lot of skipped commands, it just adds needless noise to the terminal. We still have to keep the progress bar the same length, and just remove the mtime from the total time so our ETA is still accurate.
man page: Document 'o' flag.
Add -static-gcc command for mingw.

Apparently mingw32 with gcc-4.8.1 adds a dependency on libgcc_s_sjlj-1.dll unless we add -static-libgcc now.