Something is missing. From PySideHelpers.cmake:
function(pyside_internal_detect_if_cross_building)
if(CMAKE_CROSSCOMPILING OR QFP_SHIBOKEN_HOST_PATH)
set(is_cross_build TRUE)
else()
set(is_cross_build FALSE)
endif()
set(PYSIDE_IS_CROSS_BUILD "${is_cross_build}" PARENT_SCOPE)
message(STATUS "PYSIDE_IS_CROSS_BUILD: ${PYSIDE_IS_CROSS_BUILD}")
endfunction()