Use SOURCE_DATE_EPOCH-derived date for the man page

configure.ac already turns SOURCE_DATE_EPOCH into the @DATE@ substitution, but the rule expanding xpenguins.1.tmpl still calls date(1). The .TH line of the installed man page therefore records the day of the build, so the man page - and, with --enable-selfrep, the tarball embedded in the binary - differs between two builds from the same source.

Use $(DATE) instead. Besides being reproducible, the resulting %Y-%m-%d is the date format man-pages(7) asks for, and it no longer depends on the build locale (date +%B is translated).

This patch was done while working on reproducible builds for openSUSE.