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

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index eb18eb0..a22b734 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -28,7 +28,18 @@
 #
 # Author: keir@google.com (Keir Mierle)
 
+# Avoid 'xxx.cc has no symbols' warnings from source files which are 'empty'
+# when their enclosing #ifdefs are disabled.
+if (CXX11_THREADS)
+  set(CERES_PARALLEL_FOR_SRC parallel_for_cxx.cc thread_pool.cc)
+elseif (OPENMP)
+  set(CERES_PARALLEL_FOR_SRC parallel_for_openmp.cc)
+elseif (TBB)
+  set(CERES_PARALLEL_FOR_SRC parallel_for_tbb.cc)
+endif()
+
 set(CERES_INTERNAL_SRC
+    ${CERES_PARALLEL_FOR_SRC}
     array_utils.cc
     blas.cc
     block_evaluate_preparer.cc
@@ -93,9 +104,6 @@
     low_rank_inverse_hessian.cc
     minimizer.cc
     normal_prior.cc
-    parallel_for_openmp.cc
-    parallel_for_cxx.cc
-    parallel_for_tbb.cc
     parallel_utils.cc
     parameter_block_ordering.cc
     partitioned_matrix_view.cc
@@ -123,7 +131,6 @@
     split.cc
     stringprintf.cc
     suitesparse.cc
-    thread_pool.cc
     thread_token_provider.cc
     triplet_sparse_matrix.cc
     trust_region_preprocessor.cc