Fix version date for Mac OSX and FreeBSD
Created by: gottreu
On Mac OSX and FreeBSD graphviz_version_date in version.m4 was being set to the empty string since the date command exited with an error.
Created by: gottreu
On Mac OSX and FreeBSD graphviz_version_date in version.m4 was being set to the empty string since the date command exited with an error.