Windows: Developer Build failure when copying Qt Quick Controls shaders for (+glslcore selector)

With qtbase at 285789781cd2893302edfc004d8fc07fc06b1148 , qtquickcontrols at https://codereview.qt-project.org/#/c/187682/ 751f08cf46f505980b8ebe9a228a6c229b470152 :

qtquickcontrols\src\controls

   copy /y Shaders\+glslcore\blur.frag D:\dev\5-vs15-59\qt-59\qtbase\qml\QtQuick\Controls\Shaders\+glslcore
The system cannot find the path specified.
NMAKE : fatal error U1077: 'copy' : return code '0x1'
Stop.

Basically, the argument Shaders+glslcore\blur.frag needs quotes (sth funky wrt '+' in path names).

Apparently introduced by "Enable the use of QML caching at build time" 751f08cf46f505980b8ebe9a228a6c229b470152 https://codereview.qt-project.org/#/c/187682/ , but the underlying issue is shell quoting, IMO.