diff --git a/m4/set_cflags_ldflags.m4 b/m4/set_cflags_ldflags.m4
index 08f5c983cc..23706e256b 100644
--- a/m4/set_cflags_ldflags.m4
+++ b/m4/set_cflags_ldflags.m4
@@ -15,7 +15,7 @@ for ldflag in $APPEND_LIB
 do
     APPEND_LDFLAGS="$APPEND_LDFLAGS -L$ldflag"
 done
-if [ ! -z $EXTRA_PREFIX ]; then
+if test ! -z "$EXTRA_PREFIX" ; then
     CPPFLAGS="$CPPFLAGS -I$EXTRA_PREFIX/include"
     LDFLAGS="$LDFLAGS -L$EXTRA_PREFIX/lib"
 fi