configure: fix $prefix expansion for libdir_expanded

Since the sed expansion line which did $prefix expansion for
libdir_expanded was changed from the legacy `...` format to the
new $(...) expression, the subtle backslash expansion change went
unnoticed -- \\$ which used to escape '$' now escapes '\' and the
following '$prefix' was attempted to expand as a variable. So
changing \\$ to \$ fixes this.

Also, replaced echo with printf %s -- echo does expansions of its own.

While at it, the following 2 inconsistencies were fixed:
 1) the /g flag was removed from first expression; second didn't have it
 2) first expression did not end with /, so "dropped" it from second

 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

configure

@@ -274,7 +274,7 @@ if [ -z "$LIBDIR" ] ; then
     libdir_expanded="${PREFIX}/lib"
 else
     # very non-general variable expansion
-    libdir_expanded=$(echo "$LIBDIR" | sed "s|\\${prefix}|${PREFIX}|g; s|\\$prefix/|${PREFIX}/|; s|//*|/|g")
+    libdir_expanded=$(printf %s "$LIBDIR" | sed "s|\${prefix}|${PREFIX}|; s|\$prefix\>|${PREFIX}|; s|//*|/|g")
 fi
 
 cat <<EOF