Improve ability to build custom supplements

This repo has been neglected for some time, and now I have trouble making it work. The reason for this repo was to show other developers how to create your own mymodules.so shared libraries that will work with GMT. The idea was that these modules would only require the GMT API (gmt.h) and not all the underlying lower-level library functions (gmt_dev.h) that our own supplements rely on. This repo has a minimal CMake template and setup, but over the last few years we have made enough changes to the GMT cmake setup that there are problems in this repo. For instance, this repo until recently lived in subversion, so there is all the issues with svn versioning. When I tried to build the custom shared library today (mkdir build; cd build; cmake ..) I got errors about not finding stdio.h, so something very fundamental is missing.

Separately from that repo, I think it would be great if we could extend the support we have in the main GMT CMake setup where we have EXTRA_BUILD_DIRS and EXTRA_MODULES_SUPPL. What I have in mind is the ability for developers to add a separate supplement dir to this mix, but have it build as a separate *.so file and not become part of supplements.so. This would simplify the whole build for such developers since they can simply rely on the main GMT cmake setup. How could we do this? Allow a symbolic link called custom to be placed in src (and ignored by .gitignore) and let our Cmake determine if there is such a thing then build custom.so?

Beginne mit dem minimalen CMake-Template des Repositorys und dem grundlegenden GMT-CMake-Setup, insbesondere EXTRA_BUILD_DIRS und EXTRA_MODULES_SUPPL. Überprüfe den Build, der durch „mkdir build; cd build; cmake ..“ aufgerufen wird, und wie src, gmt.h und gmt_dev.h verwendet werden. Fertig ist es, wenn die benutzerdefinierte Shared Library getrennt von supplements.so gebaut wird, ohne den gemeldeten stdio.h-Fehler.

Vom Indexierungsmodell aus dem Issue-Text verfasst.