Add better OpenMP detection and make an option to enable it.

Bug report and inspiration: http://code.opencv.org/issues/3328

CMakeLists.txt

@@ -151,6 +151,7 @@ OCV_OPTION(WITH_QT             "Build with Qt Backend support"               OFF
 OCV_OPTION(WITH_WIN32UI        "Build with Win32 UI Backend support"         ON   IF WIN32 )
 OCV_OPTION(WITH_QUICKTIME      "Use QuickTime for Video I/O insted of QTKit" OFF  IF APPLE )
 OCV_OPTION(WITH_TBB            "Include Intel TBB support"                   OFF  IF (NOT IOS) )
+OCV_OPTION(WITH_OPENMP         "Include OpenMP support"                      OFF)
 OCV_OPTION(WITH_CSTRIPES       "Include C= support"                          OFF  IF WIN32 )
 OCV_OPTION(WITH_TIFF           "Include TIFF support"                        ON   IF (NOT IOS) )
 OCV_OPTION(WITH_UNICAP         "Include Unicap support (GPL)"                OFF  IF (UNIX AND NOT APPLE AND NOT ANDROID) )

cmake/OpenCVFindLibsPerf.cmake

@@ -47,13 +47,13 @@ else()
 endif()
 
 # --- OpenMP ---
-if(NOT HAVE_TBB AND NOT HAVE_CSTRIPES)
-  set(_fname "${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeTmp/omptest.cpp")
-  file(WRITE "${_fname}" "#ifndef _OPENMP\n#error\n#endif\nint main() { return 0; }\n")
-  try_compile(HAVE_OPENMP "${CMAKE_BINARY_DIR}" "${_fname}")
-  file(REMOVE "${_fname}")
-else()
-  set(HAVE_OPENMP 0)
+if(WITH_OPENMP AND NOT HAVE_TBB AND NOT HAVE_CSTRIPES)
+  find_package(OpenMP)
+  if(OPENMP_FOUND)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+  endif()
+  set(HAVE_OPENMP "${OPENMP_FOUND}")
 endif()
 
 # --- GCD ---