Ensure support for PGI LLVM compiler

[ocrickard]

The LLVM compiler being released later this year is big news for old codes like this. It means that we can hope to target new architectures like ARM without rewriting the computational core or using tools like f2c.

http://arstechnica.com/information-technology/2015/11/llvm-to-get-fortran-compiler-that-targets-parallel-gpus-in-clusters/

When the code is published later this year, we should ensure compatibility with the source.