class Page : QWebPage {
    public:
        string test1;
}

class EnginePage : QWebEnginePage {
    public:
        string test1;
        string test2;
}

Source     +=
               one.h
               two.h
               three.h
               four.h

#if (one_defined)
    Source  += Page.h
#else
    Source  += EnginePage.h
#end

#if (one_defined)
    class Page : QWebPage
#else
    class EnginePage : QWebEnginePage {
#endif

    public:
        string test1;

#if (one_defined)
        string test2;
#endif
}

#cmakedefine QWebPage_Defined

configure_file("configure.h.in", "${CMAKE_BINARY_DIR}/configure.h")

#include "configure.h" // Include the file

#ifdef QWebPage_Defined // Warning : use #ifdef and not #if
    class Page : QWebPage
#else
    class EnginePage : QWebEnginePage {
#endif

    public:
        string test1;

#ifdef QWebPage_Defined // Warning : use #ifdef and not #if
        string test2;
#endif
}