Do not add CERES_NO_THREADS is OPENMP is not detected.

This logic was too strict and conflicted with CXX11 threads.

Change-Id: If9ac543be9765555c1eaa4e1f69d4398c72dc26f
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f0a7495..7a61c29 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -424,7 +424,6 @@
       "Clang < 3.8, and at least Xcode <= 8.  See Ceres documentation for "
       "instructions to build with LLVM from Homebrew to enable OpenMP on OS X.")
     update_cache_variable(OPENMP OFF)
   endif (OPENMP_FOUND)
 else (OPENMP)
   message("-- Building without OpenMP, disabling.")