diff -ur qtwebengine-opensource-src-5.6.0-beta/tools/qmake/mkspecs/features/functions.prf qtwebengine-opensource-src-5.6.0-beta-fix-extractcflag/tools/qmake/mkspecs/features/functions.prf
--- qtwebengine-opensource-src-5.6.0-beta/tools/qmake/mkspecs/features/functions.prf	2015-12-14 16:27:24.000000000 +0100
+++ qtwebengine-opensource-src-5.6.0-beta-fix-extractcflag/tools/qmake/mkspecs/features/functions.prf	2016-01-10 18:15:23.942032985 +0100
@@ -87,7 +87,7 @@
 }
 
 defineReplace(extractCFlag) {
-    CFLAGS = $$QMAKE_CC $$QMAKE_CFLAGS
+    CFLAGS = $$QMAKE_CC $$QMAKE_CFLAGS $$QMAKE_CFLAGS_RELEASE
     OPTION = $$find(CFLAGS, $$1)
     OPTION = $$split(OPTION, =)
     return ($$member(OPTION, 1))