Set the default domjudge_user to the current user at configure time.

Before, the use of `m4_esyscmd` would set the default to the current
user at the time configure is generated, which typically happens when
we create a release (or snapshot). The `id` command used should be
POSIX portable.

Closes #1980

(cherry picked from commit 6450b81)

Author: eldering

configure.ac

@@ -65,7 +65,7 @@ AC_ARG_WITH([domjudge-user], [AS_HELP_STRING([--with-domjudge-user=USER],
 [User that owns password files (default: current user).])], [], [])
 
 if test "x$with_domjudge_user" = x; then
-	user=m4_esyscmd([id -un])
+	user="$(id -un)"
 	# Check and warn if running as root (without explicitly setting it):
 	if test "x$user" = xroot; then
 		AC_MSG_ERROR([installing/running as root is STRONGLY DISCOURAGED, use --with-domjudge-user=root to override.])