can not use zstd compression with Qt 5.13.2 / Qt 6.4.0

The Qt 5.13.1 release notes state that "zstd" compression is not used by default, but can be used by passing "--compress-algo=zstd" to rcc.exe.

Tried adding

QMAKE_RESOURCE_FLAGS += -compress-algo zstd

and 

QMAKE_RESOURCE_FLAGS += --compress-algo=zstd

But either resulted in the error message below:

dependent 'Usage: C:\Qt\5.13.2\msvc2017_64\bin\rcc.exe [options] inputs' does not exist.

UPDATE: for Qt 5.15.0 the error message now is: "Qt\5.15.0\msvc2019_64\bin\rcc.exe: Zstandard support not compiled in".

UPDATE: for Qt 6.4.0 the error message is now a dialog showing the rcc.exe options.