Add ENABLE_<package> compile definitions for optional packages

Summary

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:

1. Is the package found in the system?
2. 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.

Related Issue

issue #252 (closed)

cmake/modules/AddMETISFlags.cmake

@@ -19,7 +19,8 @@ set(HAVE_METIS ${METIS_FOUND})
 
 # register METIS library as dune package
 if(METIS_FOUND)
-  dune_register_package_flags(LIBRARIES METIS::METIS)
+  dune_register_package_flags(LIBRARIES METIS::METIS
+    COMPILE_DEFINITIONS "ENABLE_METIS=1")
 endif()
 
 # Add function to link targets against METIS library
@@ -27,6 +28,7 @@ function(add_dune_metis_flags _targets)
   if(METIS_FOUND)
     foreach(_target ${_targets})
       target_link_libraries(${_target} PUBLIC METIS::METIS)
+      target_compile_definitions(${_target} PUBLIC ENABLE_METIS=1)
     endforeach(_target)
   endif()
 endfunction(add_dune_metis_flags _targets)