Set CMP0048 policy before project() to silence warnings (#750)

If this policy isn't set, CMake emits a large warning when project() is
called from a cmake subdirectory.

This came up when the benchmark library was added to the LLVM build, and
it was reported in https://llvm.org/PR38874. This patch was the fix I
applied locally to fix the issue, and I wanted to send it upstream.