diff --git i/Source/api.pri w/Source/api.pri
index 3b05ed1..0bd8129 100644
--- i/Source/api.pri
+++ w/Source/api.pri
@@ -20,8 +20,10 @@ haveQt(5) {

     load(qt_module_config)

-    # Make sure the module config doesn't override our preferred build config
-    debug_and_release:if(!debug|!release): CONFIG -= debug_and_release
+    # Loading qt_module_config will unconditionally add debug_and_release on mac
+    # and windows, which will generate debug and release makefiles, but we still
+    # only want to build one of them, if we ran build-webkit --release eg.
+    debug_and_release:if(!debug|!release): CONFIG -= build_all

     # Allow doing a debug-only build of WebKit (not supported by Qt)
     macx:!debug_and_release:debug: TARGET = $$BASE_TARGET