Fix problem with Solaris /bin/sh interpreting ^ in a grep statement.

r=axel@pike.org

configure.in

@@ -2815,7 +2815,7 @@ MOZ_ARG_ENABLE_STRING(extensions,
         MOZ_EXTENSIONS=""
     elif test "$option" = "default"; then
         MOZ_EXTENSIONS="$MOZ_EXTENSIONS $MOZ_EXTENSIONS_DEFAULT"
-    elif test `echo "$option" | grep -c ^-` != 0; then
+    elif test `echo "$option" | grep -c \^-` != 0; then
         option=`echo $option | sed 's/^-//'`
         MOZ_EXTENSIONS=`echo "$MOZ_EXTENSIONS" | sed "s/ ${option}//"`
     else