[CMake] Good way to add arguments to CMAKE_STRIP on macOS

[Rafe Hedley]

Hello,
I am trying to override CMAKE_STRIP to do this command:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip -x mylib.dylib

so I tried a basic :
set(CMAKE_STRIP "${CMAKE_STRIP} -x")
and 
set(CMAKE_STRIP ${CMAKE_STRIP} -x)
Unfortunately, this overriding does not work and there is the final command that I get in cmake_install.cmake :
    if(CMAKE_INSTALL_DO_STRIP)      execute_process(COMMAND "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip -x" "$ENV{DESTDIR}/Users/user/mylib.dylib")    endif()
or
    if(CMAKE_INSTALL_DO_STRIP)      execute_process(COMMAND "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip;-x" "$ENV{DESTDIR}/Users/user/mylib.dylib")    endif()
This does not work and the final command must be :
      execute_process(COMMAND "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/strip" "-x" "$ENV{DESTDIR}/Users/user/mylib.dylib")

What is the best way to override this ?
I found a really dirty trick that I can add for each target but I don't want to do that:
add_custom_command(TARGET myTarget POST_BUILD COMMAND $<$<CONFIG:Release>:${CMAKE_STRIP}> $<$<CONFIG:Release>:-x> $<$<CONFIG:Release>:$<TARGET_FILE:myTarget>>)
Thank you for your help.
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