sessreg: fix gcc6 issue with cpp output

This fix is taken from the Linux From Scratch description.

srcpkgs/sessreg/template

@@ -21,24 +21,10 @@ case "$XBPS_TARGET_MACHINE" in
 		;;
 esac
 
-post_configure() {
+pre_configure() {
 	local _gccver=$(gcc --version | awk '/^gcc \(GCC\)/ { print $3 }')
-	#
-	# gcc6 cpp fails to generate filenames.sed from filenames.sed.c
-	# thus we remove the buuld rule and provide a filenames.sed file
-	# with the expected output for Void Linux.
-	#
 	if [ "${_gccver%%.*}" -gt 5 ]; then
-		sed -i man/Makefile -e'/filenames.sed: filenames.sed.c/,+4d'
-		cat > man/filenames.sed << EOF
-/__BEGIN_UTMP_ONLY__/,/__END_UTMP_ONLY__/ d
-s|__utmp_manpage__|utmpx|g
-s|__utmp_file__|"/var/run/utmp"|g
-s|__wtmp_manpage__|wtmpx|g
-s|__wtmp_file__|"/var/log/wtmp"|g
-s|__ttys_file__|"/etc/ttys"|g
-s|__lastlog_file__|"/var/log/lastlog"|g
-EOF
+		sed -e 's/\$(CPP) \$(DEFS)/$(CPP) -P $(DEFS)/' -i man/Makefile.in
 	fi
 }