build-sys: Work around --with-rootprefix= (empty) not producing /

Since we introduced AX_NORMALIZE_PATH, using --with-rootprefix=/ does
produce an empty string, but using --with-rootprefix= (empty) now
produces "." instead which is wrong.

Work around it until we can find a better solution for AX_NORMALIZE_PATH
upstream at autoconf-archive.

Bug: #54

configure.ac

@@ -1378,6 +1378,13 @@ AX_NORMALIZE_PATH([with_zshcompletiondir])
 AC_ARG_WITH([rootprefix],
         AS_HELP_STRING([--with-rootprefix=DIR], [rootfs directory prefix for config files and kernel modules]),
         [], [with_rootprefix=${ac_default_prefix}])
+# --with-rootprefix= (empty) should default to "/" but AX_NORMALIZE_PATH
+# defaults those to ".", solve that here for now until we can find a suitable
+# fix for AX_NORMALIZE_PATH upstream at autoconf-archive.
+# See: https://github.com/systemd/systemd/issues/54
+if test "x${with_rootprefix}" = "x"; then
+        with_rootprefix="/"
+fi
 AX_NORMALIZE_PATH([with_rootprefix])
 
 AC_ARG_WITH([rootlibdir],