COMP: TubeGraphKernel. Only add -ffast-math with GCC.

Does not compile with Clang on Linux.

Change-Id: If67ae90389436b8adaebeb7df601e3a94689b9ee
Merge topic 'TubeGraphKernelClang'

8596131 COMP: TubeGraphKernel. Only add -ffast-math with GCC.