makedep: Allow building modules with C++ sources.
Used in Proton, could be useful in the future for fluidsynth and others.
The flags used for compilation are taken from CXXFLAGS + extern libs EXTRACFLAGS (which is EXTRACFLAGS without the warnings I think). Some warnings are C-only and we would need to duplicate a lot of WINE_TRY_*FLAGS to make sure whether they work or not with the C++ compiler. The other EXTRACFLAGS are assumed to work with it and it seems that they do.
Edited by Rémi Bernon