Disallow LLVM/Clang build tree explicitly; require install tree
Disallow LLVM/Clang build tree explicitly; require install tree

CastXML builds and works correctly only against a LLVM/Clang
install tree.  This is specified in the README.rst, but we
can also catch and fail early on attempts to use a build tree.

GitHub-Issue: 12

CastXML/CastXML,thewtex/CastXML,CastXML/CastXML,ellert/CastXML,ellert/CastXML,iMichka/CastXML,CastXML/CastXML,ellert/CastXML,thewtex/CastXML,bradking/CastXML,bradking/CastXML,bradking/CastXML,thewtex/CastXML,iMichka/CastXML