DLL Imports/Exports and fixes for Visual Studio Builds

[abeham]

Two changes are critical, I don't know if you want to make them:

• src/cliquer/graph.c was renamed to src/cliquer/graph2.c
• src/heap.c was renamed to src/heap2.c

Otherwise in Visual Studio (2013) both heap.c and heap.cc build to the same .o file and overwrite each other.

Other than that:

• I included PRPACK in makefile's echosources target so that these are part of the visual studio project
• I added MSDOS to the compiler's preprocessor definitions (otherwise there is a compile error)
• I removed the __BEGIN_DECL and __END_DECL directives from each header file and put them in igraph_decl.h which is included in all .h files that had these directives
• I added DECLDIR directive to all "public" functions which is either empty, or when Visual Studio is used as compiler is __declspec(dllexport) when building with IGRAPH_EXPORTS flag or __declspec(dllimport) when building without the flag

[ntamas]

Thanks for your work on this, it looks great!

I would prefer renaming src/cliquer/graph.c to src/cliquer/cliquer_graph.c and src/bliss/heap.cc to src/bliss/bliss_heap.cc instead - feels more readable than graph2.c and heap2.c.

Also, the AppVeyor build has failed for one of the build configurations because MSVC does not seem to include stdint.h:

https://ci.appveyor.com/project/ntamas/igraph/build/1.0.55/job/6x86orpkpp7d0xyf#L3494

To be honest, I don't quite understand why it fails - more precisely, I don't understand how the other three build configurations on AppVeyor could succeed if stdint.h is missing. It can even happen that prpack_solver.cpp does not need stdint.h at all. Can you try recompiling this without stdint.h on your machine?

[abeham]

Yes, very strange that only one failed. prpack_solver.cpp builds fine in VS2013 without stdint.h so I removed it. I also made the requested filename changes.

[abeham]

Okay, I think that did it. I had to avoid including stdint.h also for MINGW builds. But reusing pstdint.h was not the way to go...

[ntamas]

Thanks, now I think this one is ready for merging.