diff --git a/qwtconfig.pri b/qwtconfig.pri
index 2be4a68..8625686 100644
--- a/qwtconfig.pri
+++ b/qwtconfig.pri
@@ -7,12 +7,14 @@ VER_MIN      = 2
 VER_PAT      = 1
 VERSION      = $${VER_MAJ}.$${VER_MIN}.$${VER_PAT}
 
-unix {
-    INSTALLBASE    = /usr/local/qwt-5.2.1
-}
+isEmpty(INSTALLBASE) {
+    unix {
+        INSTALLBASE    = /usr/local/qwt-5.2.1
+    }
 
-win32 {
-    INSTALLBASE    = C:/Qwt-5.2.1
+    win32 {
+        INSTALLBASE    = C:/Qwt-5.2.1
+    }
 }
 
 target.path    = $$INSTALLBASE/lib
