regress: use gdate/gln if on *BSD To minimise the amount of churn between the difference in date(1) and ln(n) semantics, use gdate and gln from coreutils.

portable: propagate AM_CFLAGS/AM_CPPFLAGS Compiling with warnings and certain compiler flags is important for security and bug-free code. Set a bunch of warnings via AM_CFLAGS in a common Makefile, and let those values propagate to all other Makefiles. While here, also fix the version string to remove quotes.