diff -Nrup a/src/buildtools/config/functions.pri b/src/buildtools/config/functions.pri
--- a/src/buildtools/config/functions.pri	2020-01-20 12:17:16.000000000 +0100
+++ b/src/buildtools/config/functions.pri	2020-01-28 14:03:34.247189526 +0100
@@ -1,5 +1,5 @@
 defineReplace(qtwebengine_extractCFlag) {
-    CFLAGS = $$QMAKE_CC $$QMAKE_CFLAGS
+    CFLAGS = $$QMAKE_CC $$QMAKE_CFLAGS $$QMAKE_CFLAGS_RELEASE
     OPTION = $$find(CFLAGS, $$1)
     OPTION = $$split(OPTION, =)
     PARAM = $$member(OPTION, 1)