tree bd7ab5da0e6e732687619350a01b1c2e05cedc3a
parent 056140e3a005a88300c2e8bec19013111f84ec34
author Alex Stewart <alexs.mac@gmail.com> 1524404294 +0100
committer Alex Stewart <alexs.mac@gmail.com> 1524404334 +0100

Fix 'xxx.cc has no symbols' warnings.

- As OPENMP, TBB & CXX11_THREADS are mutually exclusive, and their
  implementations are wholly contained within enclosing #ifdefs if the
  requisite option is not enabled then the source file is 'empty'.
- This triggers 'xxx.cc has no symbol' warnings when compiling
  Ceres statically, as such we now only include these files if they are
  not going to be empty.

Change-Id: I5390251b44c5770bbdd4ccb1bc916dbe3b1814a9