Stop CMake from trying to detect OpenMP when using Clang.

Change-Id: Ie14c6466475b401ba35dbf13adc2e8701999c969

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c3ebff6..1ea8651 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -483,9 +483,13 @@
        "Enable threaded solving in Ceres (requires OpenMP)"
        ON)
 
-IF (OPENMP)
-  FIND_PACKAGE(OpenMP)
-ENDIF (OPENMP)
+IF (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  SET(OPENMP_FOUND FALSE)
+ELSE (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  IF (OPENMP)
+    FIND_PACKAGE(OpenMP)
+  ENDIF (OPENMP)
+ENDIF (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
 
 IF (OPENMP_FOUND)
   MESSAGE("-- Found OpenMP.")