configure: add CONFIG_DATE, GIT_CONFIG_SHA variables

Improve detecting common build errors (1)
make: pass build date and HEAD SHA during build via CFLAGS

Improve detecting common build errors (2)
rtapi/msgd: startup log with build+config dates and SHA's, warn on conflict

Improve detecting common build errors (3), and ease detecting
for logfile readers
Merge pull request #142 from mhaberler/make-clean-check

improve detecting common build errors when changing branches