Add OpenMP flags when compiling for C if enabled.

- Previously we were only adding the OpenMP flags when compiling for C++
  which could cause linker issues when compiling the C examples.

Change-Id: Ie7d8192b9da6fb17b8f554e6d164ec0a65403c99

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 63f5e10..7406fca 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -431,6 +431,7 @@
     message("-- Building with OpenMP.")
     list(APPEND CERES_COMPILE_OPTIONS CERES_USE_OPENMP)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
     if (UNIX)
       # At least on Linux, we need pthreads to be enabled for mutex to
       # compile.  This may not work on Windows or Android.