Added option for OpenMP, auto-detect was not working properly on OSX (surprise, surprise)

CMakeLists.txt

 include(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG(-std=c++11 COMPILER_SUPPORTS_CXX11)
 CHECK_CXX_COMPILER_FLAG(-std=c++0x COMPILER_SUPPORTS_CXX0X)
-CHECK_CXX_COMPILER_FLAG(-fopenmp COMPILER_SUPPORTS_OPENMP)
+
 if(COMPILER_SUPPORTS_CXX11)
 	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 elseif(COMPILER_SUPPORTS_CXX0X)
 	message(FATAL_ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
 endif()
 
-if(COMPILER_SUPPORTS_OPENMP)
-	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fopenmp")
+option( LEMMA_USE_OPENMP  "Use OpenMP in Lemma" OFF)
+CHECK_CXX_COMPILER_FLAG(-fopenmp COMPILER_SUPPORTS_OPENMP)
+if(COMPILER_SUPPORTS_OPENMP AND LEMMA_USE_OPENMP )
+	set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fopenmp -DLEMMAUSEOMP")
 endif()
 
 set(LEMMA_VERSION_MAJOR "0")