Fix math/freemat with Clang6.

Noticed by CMake 3.12 fallout. Fixed the C++ issues before I noticed that
the port was marked DEPRECATED. Porting FreeMat to llvm60 is doable, but
probably not worth it given the state of upstream. For reference:

• s/llvm33/llvm60/ in the Makefile
• OwningPtr -> std::unique_ptr
• Some Clang includes have moved around
• The real issue is dealing with how the JIT changed between

	  llvm33 and later versions.

No PORTREVISION bump, since (a) deprecated (b) compile fix for a platform
it didn't work on before and no code change where it did compile.