Non-portable configure.ac

./configure fails on my system because configure.ac does not work when 
/bin/sh is not Bash but a POSIX shell such as dash.

Please review the attached patch.

diff --git a/tmp/configure.ac b/tmp/portable_configure.ac
index 101050c..f9c5d9f 100644
--- a/tmp/configure.ac
+++ b/tmp/portable_configure.ac
@@ -168,7 +168,7 @@ if test "$enable_release_build" = "" -o "$enable_release_build" = "no"; then
 fi
 AC_MSG_RESULT([$PRODUCTNAME])
 AC_SUBST(PRODUCTNAME)
-PRODUCTNAME_WITHOUT_SPACES=${PRODUCTNAME// /}
+PRODUCTNAME_WITHOUT_SPACES="$(printf '%s' "$PRODUCTNAME" | tr -d '[:space:]')"
 AC_SUBST(PRODUCTNAME_WITHOUT_SPACES)
 
 dnl ===================================================================