Make qt_config_compile_test() delay compiling its test until the result is needed

qt_config_compile_test(integrityfb
    LABEL "INTEGRITY framebuffer"
    CODE
"#include <device/fbdriver.h>

int main(void)
{
    /* BEGIN TEST: */
FBDriver *driver = 0;
    /* END TEST: */
    return 0;
}
")
...
qt_feature("integrityfb" PRIVATE
    SECTION "Platform plugins"
    LABEL "INTEGRITY framebuffer"
    CONDITION INTEGRITY AND TEST_integrityfb
)

qt_feature("poll_ppoll" PRIVATE
    LABEL "Native ppoll()"
    CONDITION NOT WASM AND TEST_ppoll
    EMIT_IF NOT WIN32
)
qt_feature("poll_pollts" PRIVATE
    LABEL "Native pollts()"
    CONDITION NOT QT_FEATURE_poll_ppoll AND TEST_pollts
    EMIT_IF NOT WIN32
)
qt_feature("poll_poll" PRIVATE
    LABEL "Native poll()"
    CONDITION NOT QT_FEATURE_poll_ppoll AND NOT QT_FEATURE_poll_pollts AND TEST_poll
    EMIT_IF NOT WIN32
)