Only use openmp support if explicitly enabled.

--- luminance-hdr-2.3.1/cmake/CompilerSettings.cmake
+++ luminance-hdr-2.3.1/cmake/CompilerSettings.cmake
@@ -8,6 +8,8 @@
 ENDIF()
 ENDIF()
 
+OPTION(USE_OPENMP "Use openmp threading support." ON)
+
 #Activate C++11 support, when available
 if("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
     execute_process(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
@@ -39,16 +41,18 @@
 endif()
 
 #OpenMP Support under Linux, Windows with MSVC & MacOS X with GCC >= 4.3
-IF(MSVC)
-    FIND_PACKAGE(OpenMP REQUIRED)
-    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
-    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
-ELSEIF(CMAKE_COMPILER_IS_GNUCC AND UNIX)
-    EXECUTE_PROCESS(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
-    IF(GCC_VERSION VERSION_GREATER 4.3 OR GCC_VERSION VERSION_EQUAL 4.3)
-        MESSAGE(STATUS "GCC >= 4.3")
+IF(USE_OPENMP)
+    IF(MSVC)
         FIND_PACKAGE(OpenMP REQUIRED)
         SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
         SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+    ELSEIF(CMAKE_COMPILER_IS_GNUCC AND UNIX)
+        EXECUTE_PROCESS(COMMAND ${CMAKE_CXX_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
+        IF(GCC_VERSION VERSION_GREATER 4.3 OR GCC_VERSION VERSION_EQUAL 4.3)
+            MESSAGE(STATUS "GCC >= 4.3")
+            FIND_PACKAGE(OpenMP REQUIRED)
+            SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+            SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+        ENDIF()
     ENDIF()
 ENDIF()