bug#64984: call to date(1) uses non-portable flag

Hi,
In libguile/Makefile there is a call to date(1) for recording the build 
stamp as so:
   @BUILD_DATE="$${SOURCE_DATE_EPOCH:-`date '+%s'`}" ; \
    echo ' { "buildstamp",    "'`date -u +'%Y-%m-%d %T' -d 
@$$BUILD_DATE`'" }, \' >> libpath.tmp

The -d flag doesn't necessarily mean the same thing across different 
operating systems, e.g on macOS it means "Set the kernel's value for 
daylight saving time." vs the coreutils version which means "display 
time described by STRING, not 'now'"

The command errors but the build continues so it is non-fatal which is 
perhaps why it has gone undetected?



Sevan