Not block PDB file in multithreaded build on Windows.

If used cl compiler, but generator is not Visual Studio e.g. Ninja,
enable FS option to prevent blocking PDB file in multithreaded build.

cmake/OpenCVCompilerOptions.cmake

@@ -265,6 +265,11 @@ if(MSVC)
   endif()
 endif()
 
+if(MSVC12 AND NOT CMAKE_GENERATOR MATCHES "Visual Studio")
+  set(OPENCV_EXTRA_C_FLAGS "${OPENCV_EXTRA_C_FLAGS} /FS")
+  set(OPENCV_EXTRA_CXX_FLAGS "${OPENCV_EXTRA_CXX_FLAGS} /FS")
+endif()
+
 # Extra link libs if the user selects building static libs:
 if(NOT BUILD_SHARED_LIBS AND CMAKE_COMPILER_IS_GNUCXX AND NOT ANDROID)
   # Android does not need these settings because they are already set by toolchain file