bring back vimps1 that was retired in e1f9b31, clean up build process. ref #2

The use of -U_FORTIFY_SOURCE is for undefined symbols errors on xread and/or xgetcwd.

For Git, the captured child process doesn't work properly, the debug shows:

vcprompt: debug: spawning child process: git diff --no-ext-diff --quiet --exit-code
vcprompt: debug: child process git killed by signal 48
vcprompt: debug: spawning child process: git ls-files --others --exclude-standard
vcprompt: debug: child process git killed by signal 48

The modified and unknown states show correctly when the child process isn't killed with signal 48, according to kill -l, it's SIGRTMI+14.

Most of time, it's killed with the singal, few occasions that exiting properly.

See #2.