Fixed quoting in INSTALL check.

diff --git a/configure.ac b/configure.ac
index 5745914..eb91bc3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -90,7 +90,7 @@
 AC_PROG_INSTALL
 dnl Prepend src dir to install path dir if it's a relative path
 case "$INSTALL" in
-  [\\/$]* | ?:[\\/]* ) ;;
+  [[\\/$]]* | ?:[[\\/]]* ) ;;
   *)  INSTALL="\\\$(TOPSRCDIR)/$INSTALL" ;;
 esac