diff --git a/configure b/configure
index 70d5cb5..d736fb7 100755
--- a/configure
+++ b/configure
@@ -4533,7 +4533,7 @@
 # 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 $GCC in
     yes)
diff --git a/configure.in b/configure.in
index 219e925..9c8bde2 100644
--- a/configure.in
+++ b/configure.in
@@ -828,7 +828,7 @@
 # tweak OPT based on compiler and platform, only if the user didn't set
 # it on the command line
 AC_SUBST(OPT)
-if test -z "$OPT"
+if test "${OPT-unset}" == "unset"
 then
     case $GCC in
     yes)