diff --git a/configure b/configure
index 7294edc208f..1be46320650 100755
--- a/configure
+++ b/configure
@@ -4578,7 +4578,7 @@ fi
 # 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 $GCC in
     yes)
diff --git a/configure.in b/configure.in
index 7eea51f1e00..6b002c40e2b 100644
--- a/configure.in
+++ b/configure.in
@@ -863,7 +863,7 @@ fi],
 # tweak OPT based on compiler and platform, only if the user didn't set
 # it on the command line
 AC_SUBST(OPT)
-if test -z "$OPT"
+if test "${OPT-unset}" == "unset"
 then
     case $GCC in
     yes)