handle an empty OPT variable correctly #8100
diff --git a/configure b/configure
index 56e6030..90bcad4 100755
--- a/configure
+++ b/configure
@@ -4596,7 +4596,7 @@
 # tweak OPT based on compiler and platform, only if the user didn't set
 # it on the command line
 
-if test -z "$OPT"
+if test "${OPT-unset}" == "unset"
 then
     case $GCC in
     yes)