make ENABLE_ZOLTAN available to preprocessor

[JaredCrean2]

This makes the ENABLE_ZOLTAN flag passed to CMake available to the C pre-processor (via a CMake generated header). The motivation for this is trying to make code work with or without Zoltan, ie:

#ifdef ENABLE_ZOLTAN
  apf::Splitter* splitter = apf::makeZoltanSplitter(...);
#else 
  apf::Splitter* splitter = Parma_MakeRibSplitter(...);
#fi
# do stuff with splitter

[cwsmith]

We should probably call this PUMI_HAS_ZOLTAN or something like that to prevent conflicts with other packages that have a macro/pre-processor definition for zoltan.

[JaredCrean2]

Seems reasonable, although I may not get to this for a few weeks (I'm supposed to defend 2 weeks from today).

[cwsmith]

This was added as part of #342.

[bobpaw]

@cwsmith I think this can be closed.

[cwsmith]

Yes, thank you.

The pre-processor flag PUMI_HAS_ZOLTAN was added here: https://github.com/SCOREC/core/pull/342/commits/3943ce4f6cc2fcd89adeb30d9903db18a756aad1