Optional (external) packages are not linked into a module library directly, but might be linked later using cmake functions add_dune_<package>_flags or similar. In order to detect whether a package can be used, we thus have to do two checks:
Is the package found in the system?
Has the user linked the library?
The first check is done by cmake. If a find_package(<package>) is called, a corresponding <package>_FOUND variable is set. We additionally set HAVE_<package> in cmake and introduce a conditional define in the config.h file. But finding the package does not mean it is linked against a target. This can be done (for optional packages) manually, by calling the add_dune_<package>_flags macro. Unfortunately, we have no way of knowing whether a library will be linked. Thus we need to communicate this to the target using a compile flag, called ENABLE_<package>. The config.h then defines
#define HAVE_<package> ENABLE_<package>
With this MR, I introduce these missing ENABLE_<package> compile flags to some optional packages.
Simon Praetorius authored