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 $GCC in
     yes)
diff --git a/configure.in b/configure.in
index 7d14bfd..5491778 100644
--- a/configure.in
+++ b/configure.in
@@ -881,7 +881,7 @@
 # tweak OPT based on compiler and platform, only if the user didn't set
 # it on the command line
 AC_SUBST(OPT)
-if test -z "$OPT"
+if test "${OPT-unset}" == "unset"
 then
     case $GCC in
     yes)