Fix compilation when not using image plugin

[olivier-roussel]

In cases where compiling the CGALPlugin wihtout image plugin support, such as the supported-plugins SOFA CMake presets, this plugin was not compiling anymore due to the absence of definition of registerMeshGenerationFromImage method.
This PR proposes to fix this by setting a preprocessor definition when compiling with image plugin.

[bakpaul]

Your proposition is good and makes sens. The only issue is that you are doing it wrong. We want anyone building upon this plugin (which will certainly never be the case but anyway) to know if it has been compiled with it or not. To do so you need to add a #cmakedefine in the config.h.in file of the project so that a variable exists in the header. And actually the macro sofa-find_package already creates a cmake variable CGALPLUGIN_HAVE_IMAGE when image is found.

[olivier-roussel]

@bakpaul Thanks for the feedback, I agree with that and I'll make the changes.

[olivier-roussel]

@bakpaul So there is no config.h.in yet right ? we have to set up this and its configuration from scratch just for that ?
Ok I just missed it actually