doc: fix man page timestamp generating

mdate-sh cannot be used, because automake won't install it

configure.ac

@@ -10,7 +10,7 @@ AC_CONFIG_MACRO_DIR([m4])
 AC_USE_SYSTEM_EXTENSIONS([_GNU_SOURCE])
 
 # Automatically update release date based on configure.ac date
-release_date=`doc/mdate-sh configure.ac`
+release_date=`LC_ALL=POSIX date +"%d %B %Y" -r configure.ac`
 AC_SUBST([RELEASE_DATE], $release_date)
 
 # Set compiler compatibility flags

man/knsec3hash.1.in

-.TH "knsec3hash" "1" "June 2013" "CZ.NIC Labs" "Knot DNS, version @VERSION@"
+.TH "knsec3hash" "1" "@RELEASE_DATE@" "CZ.NIC Labs" "Knot DNS, version @VERSION@"
 .SH NAME
 .B knsec3hash
 \- Simple utility to compute NSEC3 hash (libknot equivalent of ISC nsec3hash)